This error occurs when you click the Authenticate button. You have not added a correct or valid trust certificate for the mail server to the cacerts keystore of the system JRE and you are connecting to the mail server over a secure protocol such as IMAPS or POP3S.
Verify that you have added a correct and valid trust certificate for the mail server to the cacerts keystore of the system JRE.
The cacerts keystore is located in the following directory: