xinetd service limit errors

I am running qmail which is being run by xinetd. When I setup my server initially, I allowed 25 smtp instances and had no issues. I then installed spamassassin, and everything still seemed OK.

Recently, however, I have been getting a lot of service_limit errors.

Jan 10 09:25:26 <hostname>xinetd[32723]: FAIL: smtp service_limit</hostname> 

I tried upping the instance limit to 30, but I am still getting these errors.

I also notice that there are a lot of sessions that are open for a very long time.

Jan 10 16:00:40 <hostname>xinetd[32723]: START: smtp pid=19885 from=77.48.174.2
Jan 10 16:01:12 <hostname>xinetd[32723]: EXIT: smtp status=1 pid=19885 duration=32(sec)</hostname></hostname> 

These all seem to be failed SMTP sessions that I am rejecting. (I am using the validrcpto patch.)

I have a couple of questions about this:

Why are these sessions taking so long?

What are the recommended number of instances for smtp I should use?

Thanks.