With default configuration of FC2's Sendmail (8.12.11-4.6), the
following appears on maillog when sending mail out to a
STARTTLS-capable server:

Jun  9 14:52:44 nasu sendmail[12145]: i59BqhBk012145:
from=<toniw>, size=297, class=0, nrcpts=1,
msgid=<1086782082.7958.23.camel@xxx>, proto=ESMTP, daemon=MTA,
relay=nasu [127.0.0.1]
Jun  9 14:52:44 nasu sendmail[12147]: STARTTLS=client: file
/etc/mail/certs/cert.pem unsafe: No such file or directory
Jun  9 14:52:44 nasu sendmail[12147]: STARTTLS=client: file
/etc/mail/certs/key.pem unsafe: No such file or directory
Jun  9 14:52:44 nasu sendmail[12147]: STARTTLS=client: file
/etc/mail/certs/cacert.pem unsafe: No such file or directory
Jun  9 14:52:44 nasu sendmail[12147]: STARTTLS=client, error: load
verify locs /etc/mail/certs, /etc/mail/certs/cacert.pem failed: 0
Jun  9 14:52:44 nasu sendmail[12147]: STARTTLS=client,
relay=the.remote.server.invalid., version=TLSv1/SSLv3, verify=FAIL,
cipher=DHE-RSA-AES256-SHA, bits=256/256

The directory /etc/mail/certs/ is also missing (and not provided by
any package).

No idea (at this point) how to fix this. One way would be to disable
STARTTLS support for outgoing mail by default. :)