If using Microsoft Outlook, please try the following:
Open Outlook.
Go to Tools > Email Accounts.
Select the Account that is causing you trouble and click “Change”.
Click on More Settings.
Click on Outgoing Server Tab.
Click "Outgoing Mail Server (SMTP) requires authentication" and make sure "Use same settings as incoming mail server" is selected.
Click on "Advanced Tab".
Set outgoing Port (SMTP) to 587.
Click OK and then Test Account Settings.
If it fails on Send, then repeat these steps:
Click on "Advanced Tab".
Set outgoing Port (SMTP) to 26.
Click OK and then Test Account Settings