[Dovecot] IMAP disconnect before greeting / banner

[Benoit Panizzon]

Hello

We have a server on which we run multiple instances of dovecot. Each instance 
serving a domain and binding to a specific IP.
The whole thing on three servers using the proxy ability and replication of 
dovecot.

Now one domain has more users and we start seeing very strange behaviour.

As soon as about 45 users have connected to the imap service ip of that 
instance, the server accepts the connections and then disconnects without 
showing the IMAP banner.
This happens on all of our three servers and only for that specific domain 
(which has most connections).

While this occurs, there is no problem with the other instances for other 
domains on the same physical machine. So the database backend etc. are not the 
issue.

While this occurs, there is no hint of what goes wrong in the logfiles. I did 
even set mail_debug=yes but that still does not produce any output of why 
clients get disconnected prior to the server displaying the banner.

Any hints on where I could start looking for the error, or get more debug 
output as with mail_debug=yes ?

-Benoit-