Why do all of my clients face a 30-60 second delay when trying to send/receive mail?

Check to make sure that the name servers that are listed in /etc/resolv.conf have not been
shut down and are working properly.

It's also very possible that the name servers in the /etc/resolv.conf file are working fine but
the ip from where the client is accessing the server is not reverse resolvable. In this case you
could turn off the reverse lookup completely.

For Linux servers the -Rt0 option needs to be added to the beginning of the server_args line in
the /etc/xinetd.d/smtp_psa file:

server_args = -Rt0 /var/qmail/bin/relaylock......

It is necessary to restart xinetd to apply the changes.

For FreeBSD servers the option is the same (-Rt0) but FreeBSD uses inetd so the modification
must be made to the /etc/inetd.conf file:

Example:

smtp stream tcp nowait root /usr/local/psa/qmail/bin/tcp-env tcp-env -Rt0 /usr/local/psa/qmail/bin/smtp.....

It is necessary to restart xinetd or inetd in order for the changes to take effect the next time SMTP is used.