[Dovecot] Disabling plus sign extension delimiter in lmtp listener (or userdb)

[Benoit Panizzon]

Hello

We have migrated our email services from a server, which did not support IMAP 
and folders, therefore threated the plus sign + as a normal character in a 
part of an email address.

Our new server delivers the emails via lmtp to dovecot.

the few users which got a + character in the username first could not log-in 
(fixed by adding + to auth_username_chars).
Now the next problem turn out to be, that the lmtp listener stripps everything 
after the + sign.

The MTA correctly sends the whole email address, so it's not the MTA's fault. 
It can easily be tested by connecting to the dovecot LMTP listener IP address 
by telnet:

220 grautvornix.imp.ch Dovecot ready.
mail from:<nobody>
250 2.1.0 OK
rcpt to:<b+enoit at iscan.ch>
550 5.1.1 <b+enoit at iscan.ch> User doesn't exist: b at iscan.ch

I could not find any configuration parameters for the lmtp listener or userdb 
service to tell it what to do with the + sign.
Did I miss something, or is it impossible to have the + sign accepted as a 
normal character in an email address?

Kind regards
-Benoit-