2. Persuade sendmail to take *all* mail without delay.
Can you explain why sendmail is slow to accept some mail? That seems
odd to me, either the mail is acceptable or it isn't, what can make
its acceptability slow?
I guess the problem is that some claimed sender domain names take one or
more minutes to fail to resolve (I've no idea why this should be). For
example:
$ time host needtodateaperson.com
Host not found, try again.
real 1m0.114s
user 0m0.000s
sys 0m0.006s
$
This one minute takes me past the time the ISP seems willing to wait, and
I think that other mails cause worse problems (I've not dug much deeper
yet).