The main difference is that with LMTP, Dovecot is in complete control over the message store: how messages are stored, how they are retrieved, per-user filters, per-user quotas, and so on. It makes a lot of sense to split the responsibilities of mail handling.
(the use of LMTP instead of SMTP is an optimization; LMTP supports
per-recipient delivery statuses, which simplifies some problems
that would otherwise require one recipient per delivery).
Wietse
