Is there a language-independent method to find out if a mailbox contains a
decline message for a given appointment? Language-independent meaning human
language independent. ;-)

The best I have come up with so far is matching the subject of all mail
objects in the mailbox with the subject line and the start date of the
appointment. This kind of string matching is not exactly
confidence-building, though, since I can't even establish that way if a
given message is a decline message for certain and a lot of false positives
show up.

Any help would be greatly appreciated.

Best Regards, Martin Schmidt.