[Dovecot] Combination of default domain and username character translation problem in POP3 server configuration

[Vico]

Hello,

I have a mail server running vm-pop3 and I am migrating it to Dovecot. I
have a problem configuring Dovecot as a POP3 server only (I'm not
interested in IMAP for now). I have two types of domains: one principal
domain, whose users don't include the domain in the login process; and
secondary domains, whose users include the domain part with the !
separator, instead of @ (i.e. prueba!mysecondarydomain.es). Because of
that, I have set the following options in the configuration file:

auth_default_realm = myprincipaldomain.es
auth_username_translation = !@

The combination of these two parameters produces the following effect
when I login with an user of a secondary domain (I have no problem with
principal domain authentication):

+OK Bienvenido al servidor de correo.
user prueba!mysecondarydomain.es
+OK
pass prueba.
-ERR Authentication failed.
quit
+OK Logging out

This is the resulting log:

Nov 19 09:58:51 prueba dovecot: auth(default): new auth connection:
pid=21953
Nov 19 09:59:04 prueba dovecot: auth(default): client in: AUTH  1
PLAIN   service=POP3    lip=192.168.1.84        rip=192.168.17.13
resp=AHBydWViYSF0b2JhcnJhLmVzAHBydWViYS4=
Nov 19 09:59:04 prueba dovecot: auth(default):
passwd-file(prueba at mysecondarydomain.es@myprincipaldomain.es,192.168.17.
13): no passwd file:
/etc/virtual/mysecondarydomain.es at myprincipaldomain.es/passwd
Nov 19 09:59:05 prueba dovecot: auth(default): client out: FAIL 1
user=prueba at mysecondarydomain.es@myprincipaldomain.es


So, I think the problem is Dovecot tests if the default domain is needed
before the character translation is done, so the character ! is not
detected as a domain separator, so the default domain is always
appended.

The question is how I could resolve this problem. I need these two
operations to be done because there is a lot of users already working
with this configuration.

Thank you in advance.