Try restarting your PC, sometimes it does the trick :), or restarting mailbird from the hamburger menu at the top left

Make sure you did not change your password recently. If you did, follow the instruction on this KB article

Disable your antivirus/firewall software and see if it helps

If you use an email that is not from a “major” provider, such as Gmail/Yahoo/etc, please try to google “Your email provider + email settings”, then change them in Mailbird.

If everything above does not work, please submit a ticket to our Support