I had to send out some mass emails, and because the site had been taken off the collective server, the only installed, configured copy of PHPList I had was on an internal server.

This server was configured, mainly to filter email and forward it to the main mail server. It was using Exim4.

The old server was (and is) using Qmail.

Exim seems to have a feature that causes mail to a domain to not be retried too often. It lets them accumulate, and then sends a few through in batches. This was a problem, because it caused the big domains like yahoo.com to get floods of email.

Qmail seems to treat each message individually (more or less) and there's less bunching-up of messages. This was what I wanted, because I configured PHPList to trickle the email out fairly slowly.

Also, what Exim deems to be a failed message is difficult to ascertain. It seems like sending a message to a nonexistent mailbox will cause that domain's email to be held up. I'll have to double-check on that sometime.