auth_username_translation and LTMP problem

auth_username_translation seems to be appliet on RCPT TO address of LTMP transport.

Why dovecot is doing that?

And better question - is there a way to disable auth_username_translation for LTMP but leave enabled for the rest (imap, pop3 etc) ?

Background:

I'm doing auth_username_translation = @=

to allow logins like aaa@bbb.pl to be internally translated to aaa=bbb.pl. That works fine.

Now my exim delivers mail to dovecot using LTMP and it does translation or it own, so it does:

RCPT TO: aaa=bbb.pl@mymbox.pl

Unfortunately looks like dovecot it doing translation one more time and looking in user database for aaa=bbb.pl=mymbox.pl where such user doesn't exist. Only aaa=bbb.pl user exists.

Thanks,

Arkadiusz Miśkiewicz, arekm / maven.pl