Hi Lissa,
451 Temporary local problem - please try later
This type of error is almost always caused by your server/host's settings and is unlikely to be a ListMail problem I can debug.
Do you know if your host has any hourly or daily limits on outgoing email? If you don't, please inquire with them about this possibility.
Does your email sometimes go out and then later, or part way, give that error? This might indicate that there are some such sending limits.
Have you changed any of your SMTP settings on the ListMail config page? Can you get
any email out via SMTP to your lists? Try a "Send Email" to a list containing a couple test (remote, not on the same server) email addresses. You may not be authenticating with the SMTP server properly.
Regards