Follow one of the following remedies to solve the problem:
1. Ensure that your Internet service provider is not blocking the port number that you are using for the outgoing mail server.
2. Note, if you're using slow Internet connection, ensure your outbox emails are not supposed to contain attachments which are too big(10 – 20 MB or more).
3. Turn off any antivirus software or disable outgoing scanning of emails by your Antivirus program.
4. Communicate with Support and make sure you have not exceeded bandwidth (not typical) or disk space.
5. Try closing and re-opening your e-mail client(Check that it's not Offline), to refresh.
6. If you are using a cell phone/ tablet, check that Mobile Data is not disabled.
If you are facing authentication issues also enable SMTP Auth see here for more info on how to enable SMTP Auth
