If you are having trouble sending emails, it's usually because of incorrect SMTP server settings, or a firewall/antivirus blocking access.
So you should try:
1. Make sure you can send that email from your web mail.
2. Go to your Identity tab (Menu -> Settings -> Identities) then double click on the account that you are having issues with. After that click the test connection button and see what error message pops up. This might point to the solution.
3. Try sending an email, with your firewall, antivirus, or proxy (if you are using this software) disabled.
4. Make sure your Outgoing server settings are correct. Just google "your email provider + SMTP server settings". Then go to the Identities from p.1 and compare the recommended settings with those you are using at the moment.
If it still does not work, please send us a support request at firstname.lastname@example.org