Recently I have had two separate test servers that were very slow sending out emails from PHP scripts via the mail() function, where PHP was configured to use a local Sendmail binary to send the emails. The scripts were hanging for roughly one minute while waiting for Sendmail to work, which was severely impacting testing efforts.
When I run a test call to mail() and tail the maillog file, I see the following entries:
$ tail -f /var/log/maillog Sep 13 11:30:27 myserver sendmail: My unqualified host name (myserver) unknown; sleeping for retry Sep 13 11:31:27 myserver sendmail: unable to qualify my own domain name (myserver) -- using short name Sep 13 11:31:27 myserver sendmail: My unqualified host name (myserver) unknown; sleeping for retry Sep 13 11:31:35 myserver sendmail: My unqualified host name (myserver) unknown; sleeping for retry
So it seems that this is a local domain name resolution issue. The trick here is to ensure that you have an entry in your /etc/hosts file for your unqualified hostname, that is the short version returned by the hostname command, which points to the local loop-back IP rather than it's external IP like so:
127.0.0.1 localhost localhost.localdomain myserver
You need to also ensure that this is the only line in that file for the 127.0.0.1 loop-back IP, as Sendmail will only look at a single line.
Once this change is made, I retested and confirmed that emails are now being sent out within milliseconds.
Updated 2022 : note that the above post was originally published in 2012, but is left here for archival purposes. The steps above have not been tested recently, so may be outdated.