[exim] Exim + Yahoo Groups = Malware?

Whenever anyone emails a Yahoo Groups listserv via my Exim server, Yahoo
Groups responds with the following: "This message has been blocked as it is
suspected to contain malware".

Repeated attempts to contact Yahoo have resulted in the unenlightening
response, "Your problem, not ours."

I am using Exim 4.88 on a Gentoo Linux server.

I have taken the following steps to try and identify the issue:

1) Telnet to Yahoo Groups and send a message manually. The message goes
through without issue, which implies that Yahoo Groups has not blocked my
IP or the email addresses impacted.

2) Send a test message from Exim to a Yahoo Group using "exim -v
listserv@???". The message goes through without issue, which
implies it is not a basic configuration issue for exim.

3) Send a test message via Exim via a telnet session to a Yahoo Group. It
works without issue, which seems to imply that the email client is at fault.

4) Send a test message from a Desktop or phone email client to a Yahoo
Group. The message is blocked and cited to be malware. Refine test by
trying a different computer, different email address, and a different Yahoo
Group, but same client -- message is blocked as malware. Refine test by
trying a different computer, different OS, different email address,
different Yahoo Group, and different email client -- message is blocked as
malware. This seems to imply that it is exim at fault, but it is not clear
how.


Since all messages sent via a mail client are being blacked as malware by
Yahoo Groups, it seems to imply that there is a greater problem, but it is
not clear at all what is going on.

Any tips on next steps to debug this issue and remedy it?

Thank you,

Jay