You're getting this error, because Mailbird detected that the certificate of the server you're connecting to, is not valid.

The most common causes include:

• The server's domain does not match the domain name in the certificate that it returned. This means that the server you are using may not be the one you wanted. It can also occur when they change their domain but don't buy a certificate for the new domain.
• The certificate expiration date is later than your system date. This may be caused by your system having the incorrect time (perhaps you are traveling and are in a different time zone), or the certificate is too old (it expired).
• The certificate has been self-signed, instead of by a recognized authority.

In all cases, you need to decide whether or not you think you are actually connecting to whatever you tried to connect to. Usually the error is due to somebody being too cheap to replace an SSL certificate when they change domains, or an error. But it could also be due to somebody trying to capture your username/password when you login. You could call them on a phone to verify that they returned the wrong SSL certificate, or browse their support forums to see if other people have the same problem.

If you decide it's nothing to worry about, press 'yes' to allow Mailbird to connect and not ask again for this server.

If you reply 'no', Mailbird will not connect to the server until the certificate has been fixed, and will not ask again until you restart.