[Dovecot] AUTH PLAIN fails while AUTH LOGIN succeeds with Exim -> Dovecot SASL auth

[Sandro Tosi]

Hello,
we're trying to setup a platform with authenticated SMTP with exim and 
relying on dovecot as auth agent.

We've read http://wiki2.dovecot.org/HowTo/EximAndDovecotSASL and so made 
the changes mentioned there, also setting

auth_mechanisms = plain login

in conf.d/10-auth.conf .

With this setup, we're able to use AUTH LOGIN method on an SMTP session, 
but using AUTH PLAIN fails.

What we receive in the SMTP session is:

auth plain `echo -e -n `\0user\0password' | mimencode`
535 Incorrect authentication data

(it's actually the output of that command that we put there), and in the 
dovecot log we see:

2012-05-24 15:04:20 auth: Debug: client in: AUTH        1       PLAIN 
service=smtp    secured rip=127.0.0.1   lip=127.0.0.1
    nologin resp=<hidden>
2012-05-24 15:04:20 auth: Info: plain(?,127.0.0.1): invalid input

Do you know where we can look to resolve this problem? If you need any 
further information/test feel free to ask.

Thanks in advance?
-- 
Sandro Tosi
Product Engineer
Shared Hosting Products
R&D | Dada.pro
eml sandro.tosi at register.it