Had to debug the SMTP connection to WebSupport SMTP server, because even the login data was correct the connection failed.
To enable SMTP logs go to Settings > General, scroll to bottom and click Config Editor…
In the config editor search for mailnews.smtp.loglevel and set it from Warn to All
To see logs go to Tools > Developer Tools > Error Console
Issue was that I had enabled GeoIP protection for my mailbox.