First find it with
$ mailq
/var/spool/mqueue (1 request)
-----Q-ID----- --Size-- -----Q-Time----- ------------Sender/Recipient-----------
w0MEs0vA055142 5289 Mon Jan 22 14:54
(Deferred: Operation timed out with some server.)
Total requests: 1
Then delete it using qtool.pl (in sendmail's contrib dir):
./qtool.pl -C /etc/mail/sendmail.cf -d /var/spool/mqueue/w0MEs0vA055142
