The Start Block determines what criteria your contact needs to meet to enter the work flow. You will determine the criteria by selecting a trigger type, and configuring your audience. 

1. Workflow trigger options.
   • Subscription event: Trigger workflow when contacts subscribe to one, many, or all contact lists.
   • Contact Property Update (CPU) event: Trigger workflow when a contact property is updated.

   • Behavioral trigger (OPEN/CLICK): Trigger when a contact opens an email or clicks a link — with optional filters by Campaigns, Tags, and Senders.

     For full details on Behavioral Triggers, jump to the “Behavioral Triggers” section of this article.

   • External event (Webhook): Trigger a workflow when your system (website, app, CRM, etc.) sends an event to Mailjet via API. You can select one or more webhook qualifiers (event “names”) to decide which external events can start the workflow.

     

     For more details on Webhooks, jump to the “External events (Webhooks)” section of this article.
2. Segment your audience (optional)
   • Select or create a segment from your existing segments.
   • Create a custom Filter: segment your audience by creating a segment that is only available within the workflow. 

Example start block configurations:

• Contact subscribed to any contact list and is located in France.
• Contacts subscribing to the contact list "Newsletter".
• Contacts subscribed to either "Newsletter" or "Promotions" and belong to an "Engaged User" segment.

• Behavioral (OPEN): Contact opens any email from campaign “Spring_Sale_2026” or “VIP_Offers”.

• Behavioral (CLICK): Contact clicks a link tagged pricing or upgrade in any campaign.

• Behavioral (OPEN + Senders): Contact opens an email sent from news@mail.example.com or offers@mail.example.com.

• Behavioral (CLICK + Campaigns + Tags): Contact clicks a link tagged shoes or sneakers in campaigns “Summer_Catalog” or “Back_To_School”.

• External event (Webhook): Contact triggers the external event purchase_completed.

Once you have configured your start block to define what criteria your audience must match to enter the workflow, you will start designing the customer journey. 

Automatically sends a predefined email to contacts upon reaching this stage of the workflow.

• Edit design: Customise your email template content.
• Campaign Name: Provide a name to identify your campaign.
• Data from webhook: The Use data from webhook option allows the Send Email block to use data received from an incoming webhook / external event. It means the email can be personalized with values sent in the webhook payload, not only with standard contact properties.

👉Example: Using data from a webhook

Imagine you have an online store. Every time a customer completes a purchase, your website sends an HTTP POST request to the Mailjet Workflows webhook endpoint. This event then triggers the corresponding action in your automation workflow.

The webhook sends this kind of data:

{
    "Tag": "purchase_completed",
    "contactMail": "localpart@domainpart.tld",
    "Data": {
        "Variables": {
            "firstname": "John",
            "product_name": "Running Shoes",
            "order_number": "ORD-45821",
            "delivery_date": "25 June",
            "discount_code": "THANKYOU10"
        }
    }
}

Your automation could be:

External event/webhook received → Send Email

In the Send Email block, you enable Use data from webhook.

Then, in the content of the Send Email block, you can make use of Mailjet's Templating Language (https://documentation.mailjet.com/hc/en-us/articles/16886347025947-Mailjet-Templating-Language) and specifically var-type variables, the replacement values for which you would be pushing with your call to https://api.mailjet.com/V3/REST/workflow2/event:

Hi {{var:firstname:"there"}},
Thank you for your order {{var:order_number:""}}.
You purchased: {{var:product_name:"your item"}}
Your estimated delivery date is {{var:delivery_date:"soon"}}.
Here is a discount for your next order: {{var:discount_code:""}}

The recipient would receive:

Hi John,
Thank you for your order ORD-45821.
You purchased: Running Shoes.
Your estimated delivery date is 25 June.
Here is a discount for your next order: THANKYOU10

Without Use data from webhook, the email would only use regular contact data already stored in Mailjet, such as contact properties which can be fetched via data-type variables (not var-type ones). It would not be able to use temporary values from the webhook, like the order number, product name, or discount code.

👉Endpoint definition

Webhook events are sent to the following endpoint:

POST /v3/REST/workflow2/event

Payload fields

If the Data object is included in the event payload, the data is temporarily stored in the contact’s workflow state. It can then be used by a future Send Email block in the same workflow.

This data is removed from the workflow state once it is used, or when the contact exits the workflow.

Fields available in the Data object

The Data object can include fields supported by Mailjet’s Send API.

Example payload

{ 
  "ContactMail": "john@example.com", 
  "Tag": "purchase_completed", 
  "Data": { 
    "Variables": { 
      "firstname": "John", 
      "product_name": "Running Shoes", 
      "order_number": "ORD-45821", 
      "delivery_date": "25 June", 
      "discount_code": "THANKYOU10"
    }, 
    "TrackOpens": "enabled", 
    "TrackClicks": "account_default", 
    "CustomID": "order-confirmation-ORD-45821" 
  } 
}

In this example:

• ContactMail identifies the contact who should enter the workflow.
• Tag defines the event name used to trigger the workflow.
• Variables contains temporary values that can be used in a later Send Email block.
• TrackOpens enables open tracking for this message.
• TrackClicks uses the tracking settings defined in the Mailjet account.
• CustomID adds a custom identifier to the message.

To use the values from the Variables object in the email, enable Use data from webhook in the Send Email block.

You can then reference these variables in the email template using Mailjet’s templating language, for example:

Hi {{var:firstname:"there"}}, 
Thank you for your order {{var:order_number:""}}. 
You purchased: {{var:product_name:"your item"}} 
Your estimated delivery date is {{var:delivery_date:"soon"}}. 
Here is your discount code: {{var:discount_code:""}}

Allows managing your contacts within specified lists:

• Subscribe to a list: Adds and subscribes the contact to a selected list.
• Add to a list and resubscribe: Adds the contact and resubscribes if already present.
• Add to a list without subscribing: Adds the contact without altering subscription status.
• Unsubscribe from a list: Removes contact subscription from the selected list.
• Delete from a list: Completely removes the contact from the selected list.

Enables updating existing properties of your contacts:

• Select the contact property you wish to update from available properties (e.g., age, city, birthday).
• Define the new value to be applied.

When using the condition block, you can evaluate the contact against criteria to determine which route in the workflow they will enter by selecting a segment. 

• The criteria for the condition is set by segments or custom filters
• Contacts that don't meet any conditions will go through a default "fallback" path.

Example:

A condition can be applied to determine if the contact has recently engaged by creating a filter using "Has opened an email in the last X days." If the recipient has opened an email sent within the last X days, they will exit the workflow. If they haven’t opened any, they will receive a new email.

The event block waits until an event has occurred before the contact moves further along in the workflow. To configure the event block you will need to select:

Event types

• List subscription — added/subscribed to a list.
• Contact property update — a property value changes.

• Behavioral trigger (OPEN/CLICK): Trigger when a contact opens an email or clicks a link — with optional filters by Campaigns, Tags, and Senders.

  For full details on Behavioral Triggers, jump to the “Behavioral Triggers” section of this article.

• External event (Webhook) — the workflow pauses until your system sends a matching external event (webhook qualifier) for that contact.
  

  For more details on Webhooks, jump to the “External events (Webhooks)” section of this article.

Configure the fallback timer: This ensures that a contact is never stuck in the workflow because the event never occurs. 

This block allows you to determine when the contact will move to the next step in the journey by setting a delay timer, custom date filter, or date property timer. 

• The Delay Timer allows you to delay the next step in the workflow by minutes, hours, or days.
• The Custom Date Timer will delay the next step based on a set date. Once you select the date, you will determine if you want it to be a one time occurrence, or happen on a recurring annually/monthly basis. 

• The Date Property Timer allows you to delay the next step based on a contact property date. (e.g., birthday).

If you want more precision on when the email sends, all three timer types allow you to specify at what time of day you would like the email to send, and on what days of the week you would like the email to send. 

The exit block represents the end of a workflow for contact. A workflow can have many exit blocks for each path represented in the workflow, and each exit block allows you to determine if the contact should be allowed back into the workflow if they match the start block trigger criteria again.

👉 404 — Contact does not exist

What you’ll see

HTTP/2 404
content-type: application/json; charset=utf-8
{"Code":"","Message":"The requested resource does not exist","Status":404,"Details":null}

Meaning
Mailjet could not find the contact referenced in your request (for example, the contact email is not present in your API key's contacts).

How to fix

• Confirm the contact exists in your Mailjet audience before sending the event.

• Verify you are using the correct account / API key (the one where the contact is stored).

• Double-check the value sent in contactMail (typos, extra spaces, wrong domain).

👉 204 — Mailjet successfully received your request

A 204 response means Mailjet successfully received your request, but it does not return a response body. A 204 does not reliably indicate whether a workflow action was triggered.

What you’ll see

HTTP/2 204

Meaning

A) Request received and the workflow action was triggered

This happens when the event matches everything the workflow expects:

• the contact exists and is identifiable by contactMail

• the webhook qualifier (Tag) matches a qualifier selected in the Start or Event block

• the contact meets any additional Start/Event block filters (segment membership, conditions, etc.)

What to do

• Check the contact’s workflow progression in the Automation UI (e.g., the contact moved past the Start/Event block, or entered the next step).

• If your workflow sends an email as the next step, confirm the message appears in message history / activity for that contact.

B) Request received, but no workflow action was triggered

A 204 can also occur when the request is valid but does not match the automation conditions, for example:

• the qualifier (Tag) does not match the qualifiers selected in the block

• the contact does not match the workflow filters (segment / conditions)

• the contact is not eligible to enter/re-enter the workflow due to workflow settings (for example, “Contacts cannot re-enter”)

• the contact is not currently waiting on the specific Event block you intended to satisfy

What to do

• Verify the qualifier spelling/casing matches exactly what is configured in the builder.

• Confirm the contact meets the segment/filters used in the Start/Event block.

• Confirm workflow entry rules (re-entry) and where the contact currently is in the workflow.

Recommendation: treat 204 as “accepted” and verify the outcome in the UI

Because 204 may correspond to both outcomes above, treat it as request accepted, then confirm the result by checking:

• the contact’s position/progress in the workflow

Verify a 204 outcome via API (Messages endpoint)

Because 204 No Content only confirms that the request was accepted (and does not provide a response body), you can validate whether the workflow actually sent an email by querying Mailjet’s Messages resource for that contact within a defined time window.  (For more details, refer to our developer documentation.)

1) Query messages sent to a specific contact

Use the contact’s ContactID and a start timestamp:

GET https://api.mailjet.com/v3/REST/message?Contact=$ContactID&FromTS=2026-01-19T00:00:00

How to interpret the result

• Messages returned (Count > 0 / Data array contains records): the workflow triggered an email send for that contact within the requested period.

• No messages returned (Count = 0 / empty Data): either no email was sent in that time range, or you need to widen the time window (or verify you are using the correct ContactID/account).

Best practices

• Use a sufficiently wide FromTS window when testing (for example, start 15–60 minutes before your test call).

• Use a consistent timezone in your logs and timestamps (UTC is recommended).

2) If you don’t have the ContactID

If your system only has the email address, retrieve the ContactID first (via the Contacts resources), then call the Messages endpoint with the Contact filter. Find more information HERE.

3) What this confirms (and what it doesn’t)

This check confirms that an email message object exists for that contact in the time range. It does not, by itself, prove which exact block triggered it—so when troubleshooting, combine it with:

• workflow UI progression (contact position in the workflow), and/or

• your system logs (timestamp + qualifier/tag sent + request GUID).

👉 400 — Invalid Tag (unsupported characters, e.g., using “-”)

What you’ll see

HTTP/2 400
content-type: application/json; charset=utf-8
{"Code":"MJ-0001","Message":"invalid Tag: should contain only alphanumeric ASCII characters and underscores","Status":400,"Details":null}

Meaning
The webhook qualifier (Tag) contains invalid characters. Tags must use only alphanumeric ASCII characters and underscores (for example: purchase_completed). Dashes (-) are not allowed.

How to fix

• Replace dashes with underscores:

  • purchase-completed → purchase_completed

• Standardize a naming convention for qualifiers across your systems (recommended: lowercase + underscores).

👉 400 — Invalid payload structure (attempting to send Tag as an array)

What you’ll see

HTTP/2 400
content-type: application/json; charset=utf-8
{"Code":"MJ-0001","Message":"error casting payload into associated model","Status":400,"Details":null}

Meaning
The request body does not match the expected schema. Tag must be a single string, not an array.

How to fix

• Send one qualifier per request (one Tag value).

• If you need to trigger multiple qualifiers, send multiple API calls—one per qualifier.

👉 400 — Invalid ContactMail (invalid email format)

What you’ll see

HTTP/2 400
content-type: application/json; charset=utf-8
{"Code":"MJ-0001","Message":"invalid ContactMail: mail: missing '@' or angle-addr","Status":400,"Details":null}

Meaning
contactMail is not a valid email address format (for example, missing @).

How to fix

• Validate email addresses in your system before calling the endpoint.

• Ensure you are sending a plain email address (no display name formatting), for example:

  • Valid: user@example.com

  • Not valid: John Doe <user@example.com>

With Mailjet’s automation builder, you can easily create workflows that trigger on recurring dates—perfect for birthdays, membership anniversaries, or renewal reminders. 

Below is a step-by-step example for setting up a birthday email workflow:

Workflow Structure Overview

The workflow consists of three main steps:

1. Trigger: When the contact's birthday is updated.
2. Timer: Wait until the next yearly occurrence of the birthday.
3. Action: Send a birthday email.

1. Define the Trigger (Contact Property Update)

Start by creating a new workflow and selecting the trigger "Contact property update".

• Set the contact property filter to birthday (date-time property).
• Apply a segment: Add a condition for “Is in list” and select the contact list you'd like to target (e.g., MyFirstTest).

2. Add a Timer (Wait Until Next Occurrence)

Add a Timer block after the trigger.

• Choose the birthday property.
• Set it to "Activate on next yearly occurrence".

• Optional: Specify a time of day, day(s) of the week, or apply a delay if needed.

3. Configure the Email (Send Email)

Add a Send Email block after the Timer.

• Create your birthday email template.
• Example subject: “🎂 Happy Birthday from all of us!”

4. Enable Yearly Looping

Connect the end of the workflow back to the Timer to ensure it repeats every year on the contact's birthday.

📝 Tips

• Run tests with dummy contacts to verify the flow.
• You can reuse this setup for other recurring events like anniversaries, renewal reminders, or loyalty milestones.