This usually means your ISP is blocking your outgoing email port. You can try to use another outgoing port to bypass the block, like port 587. But in most cases if you contact them they will just open the port up for you.
Check with your registrar and make sure that the domain name hasn't expired.
Make sure that your email inbox and/or domain didn't exceed its quota.
Use your full email address as username (i.e., firstname.lastname@example.org).
Switch to a different port such as: 587 for SMTP Outgoing Server .
Login to your control panel and remove any mail filter you have.
If you can't send and/or receive using MS Outlook/Outlook Express do the following:
Make sure that "My Server Requires Authentication" is checked in your Outlook/Outlook Express.
Make sure your Anti-Virus and/or Windows firewall are not blocking you from sending/receiving email
Go to your DOS prompt and issue the following command: ipconfig /flushdns
Run Windows Disk Defragmenter and wait until it is completed.
Power off, wait for a few seconds, and then power on your PC.
Turn off any firewall you might have on your own PC, including Norton and McAfee.
Here is a list of ports for reference:
25 - SMTP
587 - SMTP
465 - SMTP (SSL)
110 - POP3
995 - POP3 (SSL)
143 - IMAP
993 - IMAP (SSL)