Contact unexpectedly added to the group after being sent a prepared or transactional message

Overview

All email messages are linked to a group, and a contact must be a member of that linked group to receive a message.

However, three specific API calls may automatically and silently add a contact to the group if they are not already a member, instead of returning an expected error.

This can result in unintended group subscriptions, which may interfere with other group-based processes and lead to compliance issues.

Affected API calls

• message/sendSingle

• message/sendTransactional

• message/sendTransactionalWithDetails

These calls require only the recipient ID and message ID. If the contact is not part of the linked group, they are automatically subscribed before the message is sent, without any warning.

Why This Matters

This behavior violates the expectation that API calls are atomic (i.e., they only perform the specific action defined and nothing more).

It can create problems, for example:

• If the group is also used by whiteboards that trigger on membership changes.

• If a contact had previously unsubscribed, they might be re-added and receive emails they didn’t consent to.

This is different from list imports, which offer options to reject unsubscribed contacts.

Potential Legal Risks

Automatically re-adding unsubscribed contacts to a group can result in:

• Sending messages without proper consent.

• Violating data protection laws (e.g., GDPR).

Solutions

1. Use Separate Groups

Create two groups:

• One for transactional or single messages.

• One for bulk campaigns.

This way, unsubscribed contacts can still receive important transactional emails without being added to campaign groups. For example, they can unsubscribe from a main group but continue to receive transactional messages related to orders.

2. Use Segmentation to Exclude Contacts

Set up a custom profile attribute when a contact opts out of marketing messages.

• Whiteboards that send transactional messages can be set up to ignore this flag, and unsubscribed members can still get legitimate interest messages.

• Bulk campaigns must be set up to use segmentation based on the corresponding flag value of the profile attribute or check membership in an “unsubscribe” exclusion group to ensure compliance.