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. However, some servers do not work well with this setting, causing messages to be stuck in the Outbox. There are a few things you can do to fix this issue.
Turn Off Send in Background
By default, Postbox is set to send in the background, but if you have problems with messages getting stuck in the Outbox, this setting should be turned off.
Navigate to Preferences or Options > Composition > Advanced then turn OFF "Send in background."
Delete Messages from the Outbox
If a message is already stuck in the Outbox, navigate to your On My Mac or Local Folders account > Outbox and delete any messages that are not needed.
Then restart Postbox.
Repair the Outbox Folder
Navigate to your On My Mac or Local Folders account > Outbox, right-click the Outbox folder and select "Properties" from the contextual menu. Then click the "Repair Folder." button.
Compact the Outbox Folder
Navigate to File menu and select "Compact."
Wait 3 minutes, then restart Postbox.
Make Sure You Have the Correct Sent Folder Mappings
- Navigate to Preferences or Options > Accounts > [your account] > Copies & Folders
- For the Sent folder, select Other.
- Navigate to the Sent folder in that account and select it.
- 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.
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.