Sending of message failed.
The message could not be sent using SMTP server mail.2slick.com for an unknown reason. Please verify that your SMTP server settings are correct and try again, or contact your network administrator.

I got this message even though I entered in the correct username, email, incoming and out going server variable in my Thunderbird email client application. Thunderbird is supposed to auto detect the other variable, but sometimes it doesn't do this correctly.

Solution

1. You should see your email address listed in the left side of Thunderbird. Right-mouse click the email address you want to fix and click 'Settings...'.
2. Click on 'Outgoing Server (SMTP)'.
3. Select the email you want to fix and click the 'Edit' button.
4. Set the 'Port' to 25. 25 is a common acceptable port number for many major hosting providers.
5. Set the 'Connection security:' option to 'None'.
6. Set the 'Authentication method:' to 'Password, transmitted insecurely'.
7. Click the 'OK' button.
8. Click the 'OK' button.
9. Send yourself an email and see it it's working.

If your're still getting the email error, contact your hosting provider and they will walk you through proper setup.