Currently Being ModeratedNov 30, 2012 4:22 AM (in response to MaLu_Swe)
I've solved it!
After I ran an update for IPS on the Check Point Firewall and then added an exclusion for a triggered IPS definition called SMTP STARTTLS Command, the mail now works fully with the Thawte 123 certificate!
I also updated the IPS last week, but that didn't change anything.
So I guess this is a very recent change in the IPS package from Check Point.