If you are using MS Outlook, you should try this;
1) Open outlook
2) Go to Tools/Email Accounts
3) Select the Account with the trouble and click Change
4) Click on More Settings
5) Click on Outgoing Server Tab
6) Click "Outgoing Mail Server (SMTP) requires authentication" and make sure "Use same settings as incoming mail server" is selected
7) Click on "Advanced Tab"
Set outgoing Port (SMTP) to 26 or 25
9) Click Ok and then Test Account Settings