Mail Stuck in the Unsent Folder
When you send a message, Postbox sends it in the background by placing it in the Unsent folder before sending it out. This allows Postbox to close the compose window immediately. The Unsent folder is located under the Local Folders account on Windows, or the On My Mac account on macOS.
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 couple of things you can do to fix this issue.
Make Sure You Have the Correct Sent Folder Mappings!
Go to Preferences | Options / Accounts / [your account] / Copies & Folders. For the Sent folder, select "Other" then navigate to your Sent folder in that account. Restart Postbox for the change to take effect.
Verify Your SMTP Settings
Verify that your SMTP settings are correct. Preferences | Options / Accounts and on the left hand side, select Outgoing Server (SMTP).
Try restarting Postbox. Upon restart, Postbox will prompt to send unsent messages - allow it and then locate your Unsent Folder in Local Folders and verify that the messages were sent.
Turn off Background Send
Follow these instructions to turn off background send. Try sending a message via the compose window again. Notice and correct any errors that appear on send such as not being able to connect to your SMTP server.
Sending a Large Message
If you are sending a large message such as one with a large attachment, it's possible the send is still happening and is just taking a long time. In that case you can ignore this notification and allow the send to finish.
Turn off Add-ons
It could be that an add-on is giving you trouble. Try temporarily disabling them to see if this clears up the issue.
Make sure your signature does not contain images that reside behind a firewall or VPN
If you've attached an image like a company logo to your signature, 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.