Mail server down to spam attack?

Hi all,

I face a weird situation here. Our mail server cannot receive emails
from the other remote domains, except for some famous domain as
yahoomail or gmail. I can see those email come in the smtpd log, but I
never receive them. Here is the snip of my smtpd log:

2008-10-10 12:27:23.532527500 tcpserver: pid 31874 from 20x.x.x.x
2008-10-10 12:27:23.532658500 tcpserver: ok 31874
mail.myserver.com:20a.b.c.d:25 :20x.x.x.x::6081
2008-10-10 12:27:23.536745500 qmail-smtpd 31874: connection from
20x.x.x.x (unknown) to mail.myserver.com
2008-10-10 12:27:23.536751500 qmail-smtpd 31874: enabled options:
starttls  qmailqueue /var/qmail/bin/simscan
2008-10-10 12:27:44.327927500 qmail-smtpd 31874: account status is not
'localonly'
2008-10-10 12:27:44.327931500 qmail-smtpd 31874: allow relay to:
[email protected]
2008-10-10 12:27:44.327984500 qmail-smtpd 31874: mail from:
[email protected]
2008-10-10 12:28:10.072039500 qmail-smtpd 31874: rcpt to: [email protected]

If you use simscan(+clamd+spamassassin) to pipe the qmail's queue as I
do, you can understand that it only happend if you found a couple of
lines like this:

@4000000048ef00d52f83b04c simscan:[10150]:CLEAN
(-4.30/4.00):1.7578s:Re_
rose:192.168.91.35:[email protected]:[email protected]
@4000000048ef00d5302376a4 qmail-smtpd 10150: message queued:
1223622859 qp 10152 size 2278 bytes
@4000000048ef00d5303744f4 tcpserver: end 10150 status 0
@4000000048ef00d530375494 tcpserver: status: 1/80

The problem is, the second group of lines never come for pid 31874.
This problem's been starting for 3 days (as I notice). Since than, our
smtp connections can be easily drop (tcpserver: status: 40/40) so we
cannot send or receive emails; if you're curious, I've raised that
number (40) to 80 in /service/smtpd/run file. My short-term solution
is to send a term signal within 5minute interval (set via crontab) to
/service/smtpd, but I know this is not a good solution. Can you people
help me to solve this problem? Thank you very-much.

** simscan 1.4.0 scanners: clamav: 0.90.3 X-Spam-Checker-Version:
SpamAssassin 3.2.5 **

PS. Pardon my language.

Regards,