Sometimes what you see in mqueue has already tried to go out and failed for one reason or another (e.g. bad address). It tries to resend multiple times (on one mail server I have up to 5 days) then puts a failure message in the log.
You can look for the message in your mail log to see why it was deferred.
The mail sequence number is whatever follows df or qf in the file name. For example in my mqueue I have:
-rw------- 1 root smmsp 643 Jun 19 13:05 dfl5JH5TbS031352
-rw------- 1 root smmsp 973 Jun 19 13:05 qfl5JH5TbS031352
"grep l5JH5TbS031352 /var/log/maillog" shows me the following:
Jun 19 13:05:31 servername sendmail: l5JH5TbS031352: from=<firstname.lastname@example.org>, size=1010, class=0, nrcpts=1, msgid=<200706191705.NAA17877@invoice.com>, proto=ESMTP, daemon=MTA, relay=invoice.com [10.0.x.x]
Jun 19 13:05:33 atlsnml1 sendmail: l5JH5TbS031352: to=<email@example.com>, delay=00:00:02, xdelay=00:00:02, mailer=esmtp, pri=121010, relay=mail.somewhere.com. [67.x.x.x], dsn=4.7.1, stat=Deferred: 451 4.7.1 Temporarily rejected. Try again later.
The first entry is the mail as this server received it from another (this server is my relay). The second entry tells me the destination rejected the email and to try again later. Sendmail will try it again later itself.
NOTE: IPs and host names in the above have been obfuscated to protect the guilty...