If using MS Outlook, please try the following:

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 (see attached image)
7) Click on "Advanced Tab"
8) Set outgoing Port (SMTP) to 587 or 26
9) Click Ok and then Test Account Settings

If it fails on send then repeat starting from 7.

7) Click on Advanced Tab
8) Set outgoing Port (SMTP) to 465
9) Check the box Beside "The server requires an encrypted connection (SSL)".
10 ) Click ok and then Test Account Settings