I think ther should be a hint that the signature is valid but that the 
adresses don't match

No it shouldnt - actually its more a philosopical question then a 
security question.



I give you SMIME for example, SMIME v2 said - email and certificate 
email must match. SMIME v3 says, its no longer required.



The big plus for PGP was always that you are not bound to the 
certificate email address (for encrypting i.e.)



To return to your original question - lets assume you have a group 
account with multiple members but only the pgp signing key for the 
group itself (lets say support) do you think that the signature is 
invalid just because it was send by joe average from the support group 
? No. generally speaking - everyone who has the secret key is normally 
authorized to sign a message no matter which email address he uses

If I get a PGP signed message where the sender or from address is not 
the one or one to wich the sinature belongs, IMP still tells me "The 
message has been verified."

Shouldn't it complain that the from address does not match the signature?