Signature Usage stops SMTP Mail Delivery

[SysNet]

I manage a company w. SBS03 Server, using Exchange w. POP3 email accounts
from ISP using OL03SBE (not using ISA) . Using this environment for 2 years,
a week ago and no environmental changes, users starting reporting their
recipients were not receiving sent email. After checking w. their ISP (spam
blocking), TrendMicro A/V & Antispam and assuring all server/client service
packs were in place, no resolve. By chance, one user reported by removing
their signature, mail began reaching the intended recipients. I then noticed
all users use signatures, and when removed, mail always went thru and
setting back on stopped delivery. I checked Exchange badmail, spam queues
and nothing is being trapped. Any suggestions?

 

[Diane Poremsky [MVP]]

verify none of the anti span solutions enabled a jpg filter in response to
the current crop of drug spam that uses jpg's to bypass spam filters.

 

[SysNet]

Diane,
I spoke with the Trend Micro techs and double-checked settings, no blocks,
we went as far as to stop all their services and the condition continues.
Today, I was told that this is happening thru internal mail also. The
signatures are text "only", no graphics or frills, typical - Greetings, Name,
Company, Phone #. Thnx for the reply. Let me know your thoughts.