I found the following message while searching the net, which seems to be true, anyone can comment on this please?
I've noticed there are actually quite lot of domains are violating
RFCs and ignoring 550 errors. These are Exim servers (all versions)
configured by cPanel which many large hosting companies seem to be
They apparently have this auto_thaw parameter configured which
unfreezes messages stuck in the queue. Messages are frozen when they
are found to be undeliverable so usually after 4 days or after they
get a 550 error. The problem is these stupid servers will accept
bogus messages with viruses, not be able to deliver them (550 Virus
found) then try to bounce them back to the bogus sender (you or me or
some other third party). Using auto_thaw they will retry sending the
messages usually every 2 hours about 27 times over the course of 2
days despite 550 User Unknown or 550 Virus found responses... each
time the message will again be refrozen only to be auto_thaw'ed by
their stupid configuration.
By ignoring 550 errors, they are violating RFCs.
Because this usually happens without human involvement, usually virus
related, no one has every noticed or someone may wrongly think it is
an infected machine repeatedly sending viruses when it is actually
only a single message a poorly configured server is retrying to
deliver. Going through my logs after I did notice, I quickly and
easily found dozens of example systems. I want to blacklist them all,
but I'll probably won't be able to do anything more than complain