After delivering, read the Postmark response off the message: `message = UserMailer.welcome(@user); message.deliver_now; message.postmark_response` exposes the API response including the message ID, and `message.message_id` gives the standard Message-ID. Alternatively, read the `X-PM-Message-Id` hea…

Rescue the specific Postmark exception class around the deliver call: ```ruby begin postmark_client.deliver_with_template(...) rescue Postmark::InvalidMessageError => error # mark as bounced or take other action end ``` If you need finer control, inspect `error.full_response['ErrorCode']` — 406 mean…