[Dovecot] Why deliver+usercheck? deliver+MTA?

[Lukas Haase]

Hi,

I successfully configured dovecot using virtual users (and LDAP/AD). 
deliver is the LDA and verifies if the user exists (as recommended in 
the WIKI).

However, the howtos in the Wiki say *nothing* about the case that the 
recipients should be verified *before* receiving the messages (prevent 
backscatter, ...). All configurations in the dovecot-Wiki (postfix and 
exim) just accept the mails and pass them to deliver. Also, all howtos 
which I found on the web. If the user does not exist, the mail is 
bounced because the mail was already accepted by the MTA. Nowadays this 
is an unacceptable configuration!

Is there a special reason why there is no discussion about this?

However, as postfix seems to be really too unflexible I have set up exim 
to handle incoming mail and do the usercheck in the router (with an LDAP 
query). But now the user is doubled-checked: Once when receiving with 
exim and a second time in deliver. This is not necessary, so I guess I 
can disable the LDAP query for deliver and set up a static userdb.

Why does the Wiki recommened to verfify with deliver when the user needs 
to be checked at the MTA anyway?

Regards, Luke