The certificate for this server is invalid

Hi,

Our organization is using Exchange Server 2010 and mail clients include Windows Outlook, Mac mail, iPhone, iPad and Android mobile. A Mac Mail user report that he cannot receive emails and error message appear on his screen:

Mail can't verify the identity of the server "mail.lorsorsors.com".

The certificate for this server is invalid. You might be connecting to a server that is

pretending to be "mail.lorsorsors.com" which could put your confidential information at

risk.

He tried to delete all "mail.lorsorsors.com" certificates in keychain on his iMac but the same error still comes up. The other users are working normally.

Can anyone help?