This error occurs when the user running the merge has their Email Delivery Provider set to an external service (such as Outlook or Gmail), but Salesforce no longer has a valid OAuth token for that provider. When Apsona triggers the email send step, Salesforce attempts to use the external provider, detects the missing/expired authorization token, and returns EMAIL_EXTERNAL_TRANSPORT_TOKEN_ERROR, causing the merge to fail.
To fix the issue, the user must update their personal email settings to send via Salesforce instead of the external provider:
Profile Menu → Settings → Email → My Email Settings → Email Delivery Provider → Salesforce
After saving and rerunning the merge, the email runs successfully. This error applies only to the affected user, not to others running the same merge