Even simple GWIA knows it. And SMG knows because you can see the results in message tracker. So it knows the reason.

I don't disagree but, if Notify is too much, how would SMG know what text to return?

Notify is to much. Something like "smtp error 552 size limit exceeded" or "554 Your access to this mail system has been rejected due to the sending MTA's poor reputation" is better because "500 Message denied access due to content filters in effect" for any block is simple too simple (and misleading).

A message can trigger multiple filters. Should an SMTP error code really contain all that information?

How would you choose the SMTP error code and message:

• A Notify Service?
• An SMG configuration option?
• Other...

But RBL, ip reputation, size should not result in a notification service - this should be part of a simple but descriptive SMTP code. Other SMTP interfaces give good examples.

Even using the interface control service does not help because there is no further possibility to adjust ...