In LMTP when there are more than one recipient of the message, the LMTP server can report to client one response code for each recipient. With SMTP only one SMTP response has to describe the complete situation for all recipients.
Consider a case of three recipients and a virus or spam message:
- one recipient is a normal user, virus needs to be blocked;
- one is a virus lover, mail gets successfully delivered to him;
- the third one does not exist and next-hop MTA rejects it;
This clearly requires different SMTP response codes. With LMTP three response codes get returned to the MTA. With (E)SMTP it is only posible to describe the situation by:
- returning 250 success response code;
- generating proper DSN (delivery status notifications) by amavisd-new
itself, as required by standards.