error when " test connection "

When I click " test connection " after I added the mailbox. I got 2 error message :

Certificate failure for abc.com: self signed certificate: /CN=host.abcserver.com/emailAddress=ssl@host.abcserver.com

Mailhandler could not access the mailbox using these settings

anyone can suggest for what to do to fix this ? the setting are all right, nothing wrong with them.