If you're using Οutlook to send mails then check the following:

1. Click Tools then E-mail Accounts...
2. Select "View or change existing e-mail accounts" and click Next
3. Select the email account and click Change
4. Click on "More settings"
5. Select "Outgoing Server" tab
6. Tick "My outgoing server (SMTP) requires authentication"
7. Select "Use same settings as my incoming email server"
8. Click Ok
9. Press Next
10. Lastly, press Finish

Try sending mails after verifying these settings.

Same applies to other mail clients like Thunderbird.