If using MS Outlook, please try the following (it may vary slightly for other email clients):
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"
8) Set outgoing Port (SMTP) to 26.
9) Click Ok and then Test Account Settings.
Then logout from the mail client, relogin and try the same