As noted in UPDATING:
20240201:
sendmail 8.18.1 has been imported and merged. This version enforces
stricter RFC compliance by default, especially with respect to line
endings. This may cause issues with receiving messages from
non-compliant MTAs; please see the first 8.18.1 release note in
contrib/sendmail/RELEASE_NOTES for mitigations.
Here is that release note entry:
8.18.1/8.18.1 2024/01/31
sendmail is now stricter in following the RFCs and rejects
some invalid input with respect to line endings
and pipelining:
- Prevent transaction stuffing by ensuring SMTP clients
wait for the HELO/EHLO and DATA response before sending
further SMTP commands. This can be disabled using
the new srv_features option 'F'. Issue reported by
Yepeng Pan and Christian Rossow from CISPA Helmholtz
Center for Information Security.
- Accept only CRLF . CRLF as end of an SMTP message
as required by the RFCs, which can disabled by the
new srv_features option 'O'.
- Do not accept a CR or LF except in the combination
CRLF (as required by the RFCs). These checks can
be disabled by the new srv_features options
'U' and 'G', respectively. In this case it is
suggested to use 'u2' and 'g2' instead so the server
replaces offending bare CR or bare LF with a space.
It is recommended to only turn these protections off
for trusted networks due to the potential for abuse.
