Qmail automatically shutdown

[tek01]

Hi,
We have a Linux server with CentOS distribution and we use Plesk 7.5.4 control panel.

The system use Qmail as MTA, which seems to work fine except for one thing. It randomly stops itself several times a day. To get the service work again we need to manually restart it.



/var/log/messages just displays:
======================
Nov 30 13:09:12 server qmail: qmail-send shutdown succeeded
======================



/var/log/maillog gives some more information
======================
Nov 30 13:09:05 server qmail-queue: drweb-qmail[3792]: possible qmail-smtpd exited by timeout, reset connection or with "See http://pobox.com/~djb/docs/smtplf.html."
Nov 30 13:09:11 server qmail-queue: dwlib[3799]: scan: the message(drweb.tmp.RAcH3y) sent by [email protected] to [email protected] is passed
Nov 30 13:09:11 server qmail: 1133352551.548271 new msg 102137
Nov 30 13:09:11 server qmail: 1133352551.548319 info msg 102137: bytes 606 from <[email protected]> qp 3800 uid 2020
Nov 30 13:09:11 server qmail: 1133352551.604235 starting delivery 12: msg 102137 to local [email protected]
fNov 30 13:09:11 server qmail: 1133352551.604269 status: local 1/10 remote 0/20
Nov 30 13:09:11 server spamd[2505]: got connection over /tmp/spamd_full.sock
Nov 30 13:09:11 server spamd[3827]: Using default config for [email protected]: /var/qmail/mailnames/tek01.it/info/.spamassassin/user_prefs
Nov 30 13:09:11 server spamd[3827]: processing message (unknown) for [email protected]:110.
Nov 30 13:09:11 server pop3d-ssl: Connection, ip=[127.0.0.1]
Nov 30 13:09:11 server pop3d-ssl: LOGOUT, ip=[127.0.0.1]
Nov 30 13:09:11 server pop3d: Connection, ip=[127.0.0.1]
Nov 30 13:09:11 server pop3d: LOGOUT, ip=[127.0.0.1]
Nov 30 13:09:11 server imapd-ssl: Connection, ip=[127.0.0.1]
Nov 30 13:09:11 server qmail: 1133352551.755482 alert: cannot start: qmail-send is already running
Nov 30 13:09:11 server imapd-ssl: 1133352551.758259 LOGOUT, ip=[127.0.0.1], rcvd=12, sent=310, maildir=/
Nov 30 13:09:11 server imapd: Connection, ip=[127.0.0.1]
Nov 30 13:09:11 server imapd: 1133352551.762835 LOGOUT, ip=[127.0.0.1], rcvd=12, sent=308, maildir=/
Nov 30 13:09:12 server spamd[3827]: clean message (0.3/7.0) for [email protected]:110 in 0.9 seconds, 606 bytes.
Nov 30 13:09:12 server qmail: 1133352552.522281 delivery 12: success: did_1+0+2/did_0+0+1/
Nov 30 13:09:12 server qmail: 1133352552.522443 status: local 0/10 remote 0/20 exitasap
Nov 30 13:09:12 server qmail: 1133352552.522471 end msg 102137
Nov 30 13:09:12 server qmail: 1133352552.523023 status: exiting
======================

Everything starts with "Nov 30 13:09:05 server qmail-queue: drweb-qmail[3792]: possible qmail-smtpd exited by timeout, reset connection or with "See http://pobox.com/~djb/docs/smtplf.html." and a few seconds later it exits.

Why is this happening ? and how can this be fix ?