Security Alert When opening outlook

[NB]

Every first time when we open outlook we get security alerts asking us if we
want to proceed. It says there is a problem with the site's security
certificate,if select Yes or No ,will use normally and if close outlook then
open again will be show this same error.This problem occur with some
client.Why would i get that with exchange and How can I do ?
additional Information
-Server mail is Exchange server 2007
-Mail clients are MS Outlok 2007
-No Internet proxy
-OS are Win XP SP2

Thank you.

 

[Roady [MVP]]

You're using a certificate for Exchange 2007 (which is required but can be
self-signed) and it appears that something is wrong with that certificate or
the clients are not configured to trust the certificate. It all depends on
how you've set up Exchange.