This isn't a Thunderbird error message. You should talk to your email provider.

Verizon denied it was them. They said that the "mail server" was Thunderbird and said they never heard of this error message. They "proved" it wasn't them by having me send email from my verizon address to myself.

I got a forwarding email account from University of Maryland.

I should have been more clear on that. You need to talk to the University of Maryland.

Verizon denied it was them.

They have no clue.

Could verizon have made it illegal to receive to an email address that doesn't have verizon in the address? I also tested by sending from verizon account to terpalum account and got this error from postmaster@verizon: Your message cannot be delivered to the following recipients: Recipient address: drogall@terpalum.umd.edu Reason: Illegal host/domain name found

Could verizon have made it illegal to receive to an email address that doesn't have verizon in the address?

Not illegal. The Verizon server attempts to verify the umd.edu server, and that check fails. That goes for both ways, sending from your terpalum.umd.edu email address to Verizon and vice versa.

I don't think Verizon are to blame here. The problem is more likely at the umd.edu side. That's the reason you should talk to the umd.edu folks.

The answers you got from Verizon were complete rubbish though.

I reached the UMd people. You are 100% correct. They're trying to fix the issue. Thank you so much for your help!