[Dovecot] user_filter problem

[sanito at progresas.lt]

Hello,

I have a little bit complex setup. I'm trying to use dovecot lmtp and pop3
with user information stored in the LDAP database. The main problem is
that email address and pop3 username are different for every mailbox (I
cannot change this behaviour, because thousands of people already
configured their email client for such a setup). So in my situation,
user_filter for lmtp should be: user_filter=(mail=%u), but for pop3 it
should be: user_filter=(cn=%u).

Please, see:

E-mail address: john at example.com
pop3 username: 0005a
pop3 password: very_secure_password

And ldap ldiff for this mailbox:

dn: cn=0005a,dc=example.com,ou=localmail,ou=mail
uid: 0005a
mail: john at example.com
mailbox: /data/mail/example.com/0005a/Maildir/
sn: 0005a
description: testing
homeDirectory: /data/mail/example.com/0005a
cn: 0005a
userPassword:: e0NSWVBUfWxpOXBVaEw5SWJkRWc=
objectClass: inetOrgPerson
objectClass: CourierMailAccount
objectClass: top

I think I need to use different user_filter for lmtp and pop3, but it's
not possible or I cannot find how to do it. Please, help!

--
Best regards,
dammit