On 8.8.2013, at 12.58, Grzegorz Staniak <gstaniak@gmail.com> wrote:

I'm using dovecot 2.2.4 (with Postfix 2.8.4) as both the SASL authentication backend and the delivery agent, and I keep getting lines like the following in the dovecot.log from the authentication subsystem:

auth: passdb(toshiba1@domain,8.8.8.8,master): Attempted master login with no master passdbs (trying to log in as user: plain)

The connections come from an old (embedded) mail client in a printer-fax machine. I googled for this specific error, but I only found info about old clients using "user\0user\0pass" string incorrectly during authentication, which caused a problem for earlier, development versions of dovecot. Actually, I recently migrated from 1.2 to 2.2.4 exactly to alleviate this situation, but the errors persist. Is there a way to remove the problem or work around it?

So in general authentication works, but not from this client? Sounds like the client is using a broken SASL authentication. Nothing you can really do about it. Note that the above Dovecot message is an "info" message, not an error message.