> What about systems not running qmail?

Some of the dinky little Windows servers can't handle more than one
connection at a time.

For them, two simultaneous connections would be a disaster---but
_sendmail is much more likely to do that than qmail_, thanks to
sendmail's inferior queue management.

You have to hide machines like that behind an MX that does only one
connection at a time (e.g., qmail with a separate maildir2smtp queue,
or sendmail 8.8 with its one of its new options).

Larger servers have no problem with bursts of up to 5 simultaneous
connections---which, again, sendmail is much more likely to do than
qmail. Past that, mail will be delayed.

The reality is that typical overloads are caused by sustained pressure
from many hosts, not isolated bursts from a single host.

---Dan
Sick of sendmail? Don't get mad; get qmail. http://pobox.com/~djb/qmail.html