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This Sendmail error--also seen as "Transient parse error -- message queued for future delivery" or "Name server: domain: host not found"--indicates that Sendmail encountered a problem with one of its DNS resource record lookups of the destination domain.

Whenever sendmail has to deliver mails to other hosts which cannot be reached at that time, the messages are kept in the queue. Although the other hosts could be reached again and you want to tell sendmail to flush the mail queue, the command

sendmail -q -v

does not really try to reconnect to these hosts and still assumes that the connection timed out. The reason is that the hoststatus is cached, per default for a period of 30 minutes. Using

sendmail -OTimeout.hoststatus=0m -q -v

you can re-run the mail queue and force sendmail to reconnect to the hosts.

Sendmail does not provide a command-line argument to remove messages from the mail queue. It may be necessary to manually remove messages from the mail queue rather than allowing Sendmail to attempt redelivery of messages for Timeout.queureturn days (5, by default).

The proper way to remove messages from the mail queue is to use the qtool.pl program included in the contrib subdirectory of the Sendmail source code distribution. qtool.pl uses the same file locking mechanism as Sendmail...

Adjusting sendmail queue time / lifetime

By default, sendmail will keep items in the queue for up to 5 days. If you want to make this something shorter, like 3 days, you can adjust the following in /etc/mail/sendmail.mc:

Sending user names and passwords across the Internet in plain text is bad. Fortunately, sendmail provides another method of encrypting not only the user name and password, but the entire mail transmission, using Transport Layer Security (TLS)--which is only slightly different from its predecessor, Secure Socket Layer (SSL) version 3. Sendmail uses the STARTTLS command within an SMTP conversation to signal the beginning of an encrypted TLS conversation...

Some collected tips and tricks for sendmail and related mail utilities. For more documentation, see the cf/README, sendmail/SECURITY, and doc/op/op.ps files under the sendmail source...

Sendmail has always been a bit like 'black magic' that works best when left to it's own devices. The rising tide of spam emails however is causing trouble that sometimes requires intervention to track down the cause of mail jams and sometimes even to manually remove emails from the server...