I've been using a free mailing list script called PowerList for the past 3 years, and it's worked well.

There are less than 400 people on the mailing list.

Today I sent out an email and about 100 of them weren't sent.
I got an error message saying that
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

==name removed++@msn.com

unrouteable mail domain "msn.com" .

These 100 emails that weren't sent were at the end of my email list.
It is a text file.

Is this something with my webhost? Like that they're throttle back on the number of emails that can go through?

This script is supposed to work and has worked.

Or can someone suggest a good alternative.

Thanks.

The unrouteable mail domain "msn.com" error would seem to indicate an issue with mail delivery to that domain, which likely means their routing is screwy in regards to DNS lookups, or their DNS servers. Possibly, it's another issue, such as a broken email configuration/server.