Per Ken Cormack:
If you impliment greylisting with MIMEDefang,
you MIGHT want to disable sendmail's support for PIPELINING.  (See rfc 2920:
SMTP Service Extension for Command Pipelining)
 
You can check your sendmail to see if it currently supports pipelining, as
follows:
 
sendmail -d0.1 -bt < /dev/null
 
If you see PIPELINING in the "Compiled with" options, then pipelining
support is active.  To disable support for pipelining (which is enabled by
default if you compile the sendmail source tar-ball), you need to add the
following statement to your devtools/Site/site.config.m4 and recompile
sendmail.
 
APPENDDEF(`conf_sendmail_ENVDEF', `-DPIPELINING=0')
 
Ken


Per Chris Myers:

PIPELINING can also be disabled in the sendmail access database file with
the srv_features item.  Just add the following line to your /etc/mail/access
file and rebuild the access database:

srv_features:                   P

(the whitespace is composed of TAB characters, NOT spaces!).

If you need to, other options include:

    A    suppress SMTP AUTH
    S    suppress offering STARTTLS
    V    turns off request for client certificate during TLS handshake

If you have more than one option, use commas to separate the values.  You
can also control this per-host or per-domain, with

srv_features:host.domain    options
srv_features:domain    options