Service Not Available

["Jon Daily" <jonfdaily@gmail.com>]

Hello,

I've had my qmail server up and running for awhile now, and it is
working.  However, I know when I am sending messages to it through my
application that I often get "Service not available. Reply: " messages
for what seems like no reason.  This isn't a huge problem for my
application because it just keeps on trying to send the message until
it is accepted.

The problem is that other mail servers usually don't attempt to resend
it immediately, but usually wait a substantial amount of time.
However, this lag is unacceptable for the application.  I have been
able to dramatically decrease the "Service not available" messages by
adding four MX records all pointing to different IP addresses that
actually point to the same qmail smtp server, but I still think this
response is happening too frequently.  I am currently handling a small
amount of messages ( < 1/min) and am worried what will happen when the
traffic grows.

Does anyone have any idea why I may be receiving this, or a possible
fix.  Is this normal?  Do I need a fifth MX record?

Thanks much,

- Jon