If your Mailbird decided to stop receiving or sending email all of a sudden, it could be caused by one of this thing.
1. A new security software is blocking access to Mailbird
2. You have a new proxy connection
3. You recently changed your password
4. You have another email client set up as POP3, which takes over your emails before Mailbird
Here's what you can do to solve the issue
If it's caused by number 1 or 2, try to disable your anti virus, or proxy connections, and then try to connect to Mailbird again. If you can receive email again, you need to add an exception for Mailbird in your security software.
If it's caused by number 3, you need to go to Mailbird settings, navigate to the accounts tab, double click on your account and change the password manually.
After that do the same but with your Identities tab
If the issue is still there, please contact our support at firstname.lastname@example.org so we can troubleshoot the issue further.