using exim on centos, with cpanel/whm, dedicated server, not used for hosting, only a couple of friend's sites and my own.
yeah i assumed 800 pales in comparison to some out there, but i've seen emails being delayed in being sent (tested sending one, saw it in in the queue) which is why i wanted to do something about the queue as it seems to be the cause.
If so make sure that you config qmail to NOT do DNS lookups. This can drastically increase the time for logging in and sending mail.
this may or may not apply to your situations, but this is from the "Life with qmail - Gotchas secton"
G.9. qmail is slow if /var/qmail/queue/lock/trigger is gone or has the wrong permissions or is set as a regular file. qmail-queue and qmail-send communicate via a named pipe called /var/qmail/queue/lock/trigger. If this pipe gets messed up, qmail-send doesn't notice new messages for a half hour or so.
The best way to ensure that it's set up right is to run "make check" from the source directory. If that's not possible, make sure it looks like:
make sure theres a P at the beggining of the line, this lets you know its a named pipe, make sure the mode (especially world writable), and the owner/group are all as above.
G.10. DNS or IDENT lookups can make SMTP slow
If qmail-smtpd is slow to respond to connections, the problem is probably due to DNS reverse lookups or IDENT lookups. If you're starting qmail-smtpd with tcpserver, remove the "-h", "-p", and "-r" options and add "-H", "-P", "-R", and "-l 0" (ell zero).
Also, Be sure to watch your /var/log/qmail/smtpd/current for problems involving
not having enough memory. You may need to increase the softlimit memory
size in /service/qmail-smtpd/run.