If you receive an error message while you try to send the email
Most likely there is a communication problem between Outlook and your outgoing mail server, so the email is stuck in Outbox because Outlook can’t connect to your mail server to send it.
1. check with your Internet service provider to make sure it doesn’t block the port number that you are using for the outgoing mail server (some providers block port 25 or other ports that may be used by an outgoing / SMTP mail server);
2. check with your email address provider and make sure your mail server settings are up to date. Occasionally, mail server providers change port numbers or authentication methods, so even if your Outlook account used to send emails without any problems, it may not be working anymore because the outgoing mail server settings were changed;
3. check with your email address provider and make sure you didn’t exceed the hourly / daily email send quota (or other send limits enforced by your email provider);
4. if you are on a slow Internet connection, make sure your Outbox emails are not supposed to include very large attachments (10 – 20 Mb or more). The server connection may time out before Outlook gets to send a large email;
5. if the above solutions don’t work, use Google and search for the exact error code that you received while you are trying to send the Outlook email: it will likely lead you to a precise solution.