Exchange 2007 Event Id : 12018

What you need to do next depends on what type of certificate it is.

If it is a commercial SSL certificate then you just need to replace it.
If you have used the self generated certificate then you need to start to plan how to replace it.

The best way would be to acquire a commercial SSL certificate. That will avoid the need to visit the users, as long as they are using a valid name in their RPC over HTTPs/Outlook Anywhere configuration.

Trying not to sound like my mother, but if you had deployed Exchange correctly, using a commercial SSL certificate then you wouldn't have this problem. Considering you can get SAN/UC certificates for less than US$100/year, trying to use self generated certificates is a false economy.

The certificate on both the Exchange server and the ISA server should match.

As already mentioned you should be using a UCC/SAN certificate with Exchange 2007 and the certificate should include:

autodiscover.domainname.com
owa.domainname.com (owa URL)
servername.domainname.local (internal FQDN of the server)
SERVERNAME (NETBIOS name of the server)

see my guide here on publishing Exchange with ISA:  http://demazter.wordpress.com/publish-exchange-services-with-isa2006/

have a look here for further reference: http://www.exchangeinbox.com/article.aspx?i=114

In the event ID 12018, yu would sethe certificate thumbprint mentioned
Run the below command

Get-exchangecertificate -thumbprint (Thumbprint mentioned in event 12018)

Then note the certificate is for SMTP, POP and which all services.

Then run the following command

Get-exchangecertificate -thumbprint (Thumbprint mentioned in event 12018) | new-exchangecertificate

Then it will prompt you if you are sure you would like to overwrite  it say yes

Refer the below artilcle for any queries

http://exchangepedia.com/blog/2008/01/exchange-server-2007-renewing-self.html

hi shivkumar
excellent, i have done it before itself, but your answer was exacting matching to our requirements
keep it up, have a nice day
regards
Jinesh

excellent

You are welcome sir, just to add run iisrest from command propmt after that.