It would help a lot if we knew what error messages that sender was receiving. Are you seeing them in your online account? If not, then thunderbird is not even involved.

I had them try again. Here is what they got:

From: Mail Delivery Subsystem <mailer-daemon@googlemail.com> Date: Mon, Feb 20, 2023 at 12:06 PM Subject: Delivery Status Notification (Failure) To: <lisa@reimageamerica.com>

Error Icon Message blocked Your message to cgille@reagan.com has been blocked. See technical details below for more information. The response from the remote server was:

554 5.7.1 <mail-lf1-f42.google.com[209.85.167.42]>: Client host rejected: The sender's mail server is blocked by this recipient

The message says that you are blocking it at the server level. Check your online spam settings. This happens before THunderbird sees the message.

How/where do I check online spam settings? Not the browser, right? I don't know where to look.

Your problem has nothing to do with Thunderbird. Thunderbird can only access The server has blocked the email. So you need to logon to your webmail account via a browser. Then you need to check out the spam settings. Maybe there is a filter which has been set up to refuse any mail from the sender.

It may also be worth checking to see if you have reached your quota. Servers allow a certain amount of space for emails that's all emails, both sent and received. If you have reached a max quota allowance then the server will not accept any emails sent by anyone until you have created some space on server. So if you delete a few emails and create a bit of space then a few emails will get accepted by server. You may need to delete a load of old emails off the server to get everything working normally.

I'm at a loss and I sent a request to some others who may have more insight.