The script in /etc/init.d (sendmail) will read this file for the options.
-db starts sendmail as a daemon, -q5m says flush every five minutes. Note that most mail is sent and forwarded immediately, flushing is needed only if for some reason the route to intended recipient (and intended mail exchangers) is down (most usually for me, my own internet connection), or if you are acting as a MX for another mail server. In those cases, mailed is spooled in /var/spool/mqueue until it either timesout (usually about 5 days, depending on setup) or can be sent on.
When submitting mail by using sendmail as a mail submission program, sendmail copies all messages to "/var/spool/clientmqueue" first. Sendmail is a setgid smmsp program and thus gives any user the permission to do so (/var/spool/clientmqueue belongs to user and group smmsp). Later, another sendmail process, the sendmail mail transfer agent (MTA) copies the messages from /var/spool/clientmqueue to /var/spool/mqueue and sends them to their destination.
/var/spool/clientmqueue is thus the holding area used by the MSP (Mail Submission Protocol) sendmail instance before it injects the messages into the main MTA (Mail Transport Agent) sendmail instance.
Sendmail will save the message in /var/spool/clientmqueue for safe keeping before trying to connect to the MTA to get the message delivered. Normally there would be a 'queue runner' MSP sendmail instance which every half hour would retry sending any message that couldn't be sent immediately. Each message will generate a 'df' (message routing info) and 'qf' (message headers and body) file. You can list out all of the messages and their status by:
# mailq -v -Ac
When files accumulate in /var/spool/clientmqueue, this is probably due to sendmail localhost MTA not running, and thus the mails don't get send.
The E-mail Message's Way from Client to Client
After you have sent the e-mail message to the SMTP server that communicates with your computer the message still has a long way to travel before it reaches your recipient's computer. The moment after the e-mail message has been successfully transferred to your SMTP server; the server must find your recipient's mail server and transfer the message to that server. To locate the server that the recipient uses, the recipient's e-mail address will be examined and the domain name extracted by reading the information after the "@" character (the domain name is always found after the "@" character in the e-mail address). By using this domain name and then checking with a Domain Name Server (DNS), the location (the IP number) of this server will be found. After that, a communication line will be established between your mail server and your recipient's mail server.
The communication between these two mail servers are basically the same as the communication between your computer and your SMTP server. The SMTP protocol is used and the port number 25 (at your recipient's mail server) is used. The commands used are normally also the same and the e-mail message is transferred in similar way as when it was transferred from your computer to your SMTP server.
However, sometimes there can be technical problems at the recipient's server and the e-mail message can not be transferred. To handle such a situation the message goes into a queue. The SMTP server on most machines uses a program called sendmail to do the actual sending, so this queue is called the sendmail queue. Sendmail will periodically try to resend the messages in its queue. For example, it might retry every 15 minutes. After 4 hours, it will usually send you a piece of mail that tells you there is some sort of problem. After 5 days, most sendmail configurations give up and return the mail to you undelivered.
If the message have been successfully transferred to your recipient's mail server, it will then be stored in his/her POP3 server and when the recipient start downloading his/her e-mails, your e-mail message will appear in the inbox of the recipient's e-mail client program.
# service sendmail stop # cd /var/spool/mqueue # for ML in `grep -l "misspelled_address\@gmail.com" *`; do sed -i -e 's/misspelled_address\@gmail.com/postmaster\@localhost/g' $ML; done # service sendmail start # sendmail -q -v