Mail Stuck in the Unsent/Outbox Folder

When you send a message, Postbox sends it in the background by placing it in the Unsent/Outbox folder before sending it out. This allows Postbox to close the compose window immediately. The Unsent folder is located under the On My Mac account on macOS, or the Local Folders account on Windows.

Postbox monitors the Unsent folder to make sure messages are not getting stuck there and not getting sent out. A notification bar will appear in Postbox if it detects this situation. There are a few things you can do to fix this issue.

Make Sure You Have the Correct Sent Folder Mappings!

  1. Navigate to Preferences or Options > Accounts > [your account] > Copies & Folders
  2. For the Sent folder, select Other.
  3. Navigate to the Sent folder in that account and select it.
  4. Restart Postbox.

For more details, please see Folder Mappings.

Verify Your SMTP Settings

Verify that your SMTP settings are correct in Preferences or Options > Accounts > Outgoing Server.

Delete Messages from the Outbox 

Navigate to your On My Mac or Local Folders account > Outbox and delete any messages that are not needed. 

Then restart Postbox.

Sending a Large Message

If you are sending a large message, it's possible that the message is just taking a long time to send. In that case, ignore the notification and allow the send action to finish.

Make sure your signature does not contain images that reside behind a firewall or VPN

If you've attached an image to your signature like a company logo, make sure that it is not behind a firewall or VPN, and that the link to this image can be publicly accessible. Try loading the image at home when not connected to a VPN.