If you are AOL/ Yahoo user, you might not be able to log in Mailbird although you know the password is correct. 

Here's the solution:

1. Sign in to AOL via the website/app 
2. Click Account security. 
3. Turn on Allow apps that use less secure sign in.

That's it.

If you still cannot add your account, it can occur for multiple reasons and it is because something is blocking Mailbird to open connection on port 80. You can either try to find out what is it and fix it or use Username+Password instead of OAuth.

If you wish to use Username+Password method, instead of using your account password you will need to use app password that can be generated on Yahoo website under "Settings -> Account security -> Generate app password".

In order to fix the OAuth issue with port 80, please follow steps in this article: https://appuals.com/fix-an-attempt-was-made-to-access-a-socket-in-a-way-forbidden-by-its-access-permissions/.

If none of those worked then most likely other application is blocking port 80. Most common cases are: Skype, Team Viewer, IIS, Exchange Server and SQL Server Reporting Services. If you have any of those, close them and try again.

If none of the above worked, please follow these steps to find out what app is using port 80:
1. Open Command Prompt and run netstat -ano | find ":80"
2. Check if you see any "0.0.0.0:80" entry and if so copy value of the last column from the results (PID of the application that is using port 80)
3. Open Task Manager on Processes tab, right-click on any column header and enable PID column to be visible.
4. Find the entry with the PID number found in point 2 and check what is it.
5. If it is not a "System" but some third-party app you can just end the process and you should be done.
6. If this is "System" though you need to check what part of System is it exactly. In order to do that:
A) Run telnet 127.0.0.1 80 command (you might need to add telnet feature to Windows if it is missing: please follow this: https://bwit.blog/telnet-is-not-recognized-as-an-internal-or-external-command/) and if you see black screen type "GET" and hit enter.
B) If you see in the results anything with Microsoft-HTTPAPI/2.0 inside, please run net stop http command
C) You will be presented with the list of services that are depended on HTTP Service. Please open Services from Administrative Tools in Control Panel and go one by one with those services and stop it (right click -> Stop) and check if that fixes the issue in Mailbird. Stopping one of those services should solve the issue.