It is done by spamassassin.
The actual subject is set in /etc/mail/spamassassin/local.cf but if you have qmail-scanner then it is set in /etc/qmail-scanner.ini and from the subject you posted it looks like you have qmail-scanner installed.
As to what to do to resolve this, well, the first thing to do is find the cause. In /usr/local/psa/var/log/maillog try to find the entries relating to the email being sent, to see if anything is logged.
Some things that may be the cause include their public IP being on one or more blacklist or dynamic list, or being treated as such at least.
in local.cf you can add things like whitelist_from (e.g. whitelist_from *@domain.tld
) which will whielist email "from" that particular sender's address. Of course if a spammer fakes their address and sends them spam, it won't be flagged.
You can also add things like trusted_networks aa.bb.cc.dd to trust IPs or subnets
There is a long thread somewhere -- either here or on the parallels forums, on attempts to stop spamassassin treating any email sent via authenticated smtp from being marked as spam. As I recall, the suggestions didn't work. I certainly never managed to make them work at any rate.