FC7 - using FC5 sendmail.mc - and all mail going into blackhole

[B Wooster]

Looking for tips on how to debug this problem.

I use SMART_HOST to relay my mail to smtp.rcn.com - and this worked
fine under Fedora FC5.

I just installed FC7 (clean full install, not an upgrade), copied over
my SMART_HOST commands, as well as MASQUERADE lines from the working
FC5 sendmail.mc, re-created sendmail.cf, restarted sendmail.

But all mail I send never gets to its destination - best I can tell,
smtp.rcn.com says "accepted message", but then just drops it silently.
/var/log/maillog shows:
Aug 20 19:44:01 MYHOST  sendmail[3310]: l7KNi1AT003308:
to=<bwooster47 at gmail.com>, ctladdr=<USER at localhost.localdomain>
(500/100), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=120391,
relay=smtp.rcn.com [207.172.4.99], dsn=2.0.0, stat=Sent (ok:  Message
455170823 accepted)

But that message never makes it to my account at gmail.

I then tried using the KMail program - which bypasses sendmail - set
it up for Sending using SMTP, and entered same host - smtp.rcn.com
Bingo - all email works from kmail.

So, smtp.rcn.com accepts my sendmail messages, and throws it away.
kmail smtp.rcn.com accepts the mail, and processes it correctly.

Maddening... what could be wrong with sendmail setup? Would have been
better if rcn.com behaved better and replied with an error if it was
going to drop email... but that is something totally out of my
control... any ideas on what to tweak in sendmail so it looks just
like kmail to smtp.rcn.com?