Certificate expiry problem with Mail

My place of work requires SSL to collect email from their exchange server. The certificate I had been using to collect mail from there for the past few months has expired, and Apple Mail will no longer collect mail because of this.

However, our technical support people claim that the certificate on the server has been replaced with a new one, and I have been able to confirm this by logging in correctly from a different user account on the same Mac.

I therefore conclude that the outdated certificate is being cached somewhere within the user-specific settings on my proper Mac user account. It doesn't show up anywhere in Keychain Access, I've checked every known keychain.

Can anyone suggest a solution to this (or a better explanation if I'm wrong) ?

Our tech support people are just laughing at the mac, saying I should use a proper computer. I am in danger of having the mac forcibly removed and a PC put in its place. Please suggest anything I might try!

Thanks,

-Jef.