If you're experiencing issues connecting to a Yahoo account, sure your username and password is input correctly and is receiving this error:
"[AUTHENTICATIONFAILED] AUTHENTICATE Invalid credentials"
It may be due to a new security feature in Yahoo that disables access from what they call "less secure apps". It does not mean Mailbird is not secure - we connect using full encryption - it's just what Yahoo calls it when a non-Yahoo application connects using a username and password.
To be able to connect to Yahoo, enable access by following the steps at the bottom of this guide:
You should now be able to connect via Mailbird.