Is there a way that I can distinguish null sender verification failures?
I'd like to customise the error message depending on the reason for the
callout failure. These are the conditions I'd like to detect:
1. temporary failure
2. rejection after MAIL FROM:
3. rejection after RCPT TO:
In particular, I'd like to reject in condition (3) with an error that says
something like "your mail server says you don't exist", in condition (2),
I'd like to do something else.
