"message Frozen" warnings from outgoing mail server

[#1]

Since about a week ago I've been getting reports from relay.plus.net saying that certain messages are frozen, because of spam or virus.
The messages in question are being sent to my server at home, which is on 24/7 connected to the ADSL line. For certain organisations for which I run web sites I have email sent to special addresses here, processed with spamassassin and (if not rejected) sent on to their proper destination.
I accept that my spam filtering may be missing some email-borne viruses,  but this system has been working fine for years, only suddenly now I'm getting these reports from relay.plus.net which I never got before. Has something changed?
I'm quite sure that spam and viruses are not originating from my system.
Anahata

[#2]

Have you run multiple Anti Virus systems and malware checkers to make sure there's nothing buried in your computer?
Might want to change your password to the email account to afterwards to be positive everything is secure.