IE7 not saving a self signed SSL cert

[Burble]

A few weeks ago I migrated my email platform over to Exchange running on my ESX server. I've setup OWA (and POP3 + IMAP4) using a self signed SSL certificate.

Thunderbird has accepted and saved the certificate even though it isn't valid (remember, it's self signed) but IE7 refuses to do the same. Every time I connect to OWA it warns me that the certificate is not valid so I have to click the link to continue to the site.

Next to the addressbar is another bar telling me about the certificate error. If I click that and choose 'view certificates' then I have the option to install the certificate. I click that and go through the import wizard then get a message saying that the import was succesful but each time I connect to OWA it tells me that the certificate isn't valid.

Any ideas? it doesn't bother me because I rarely use OWA but my Mother and Brother often do.