I started getting compains about mails returning with 5.1.2 error. And now I started getting returned mails very often.

Hi. This is the qmail-send program at xxxxx.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

:
Sorry, I couldn't find any host named xxxx.com. (#5.1.2)

why does this happen ? why now ?

you need to supply more info to best help you out.

what version of plesk are you using?

how many domains on your box?

have you noticed the email flowing out to increase steadily? or all of a sudden a lot of email one day?.. (one day not many emails going out - then another day, you see a big increase - a lot more than usual)

do you have formmail on any accounts?
run command at root/su - : locate Formmail, locate FormMail and locate formmail and see if there are any formmail scripts you don't know about.

The best suggestion here, is to get a program called ISOQLOG, http://www.enderunix.org/isoqlog/ This can help you see what all accounts are sending/receiving.

IHTH,
Peace.

plesk version is 7.02
qmail rpm's are:
psa-qmail-rblsmtpd-0.70-rh9.build040430.16
psa-qmail-1.03-rh9.build040430.16

with qmail-scanner:
qmail-scanner-1.22-art.rh9.4

qs is only checking for viruses only and not spam...

The machine conf is P4 2.0 ghz. 512 mb ram.
About 150-160 domains being hosted.

iptables is running as firewall. only service ports are open and higher ftp ports for passive transfer.

concurrencyremote is 100

I have increased concurrency to 600 and lowered it back to 100 as this might have caused the failure in sending mails.

only 1 formmail found in all hosts. load average varies from 0.4 to 0.6.