Here I have mentioned some points that you should keep in mind to keep yourself protected from Outlook mail stuck in outbox issue.
1. Verify your Internet service provider to make assured 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 assured your mail server settings are up to date. Irregularly, 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 overdo the hourly/daily email send quota (or other send limits enforced by your email provider);
4. If you are on a late Internet connection, make sure your Outbox emails are not assumed to include very large attachments (10 – 20 Mb or more). The server connection may time out before Outlook gets to transmit a large email;