SMTP Error 512 : This SMTP reply code is received when one of the servers on the way to the destination is unable to resolve the domain name of a recipient email address. Said differently: one of the servers on the way to the destination, including your server or your ISP, has a DNS problem or, possibly correctly, does not like one of the email addresses in the message’s TO, CC, and BCC fields.

Most SMTP error 512 conditions are caused by misspellings of the domain name part of a recipient email address.

I suspect it's something within the setup of Thunderbird since I can switch to a different computer and send via Apple Mail without a hitch. Host service HostGator no help.

Using a different computer proves very little except there are no typo errors on that computer.

Are you sending one email address in a TO field ? Are you sending a Mailing List? Create the email and post an image. Send email. When you get the Alert message, post image showing the details of what was rejected.

I am attempting to send then the Alert message appears and the message does not send in any way shape or form, simply remaining on the screen unsent.

I am simply sending, in Thunderbird, FROM my verizon.net address/account TO my e-mail account hosted by HostGator. No groups, just one-to-one.

The Alert image was attached to my original post and I will attach again in this reply. That's all that appears over the idling message.

It would have been useful to be able to see the domain it's complaining about.

Since I can execute the same send from one to the other on another computer using Apple Mail, the trouble is not in the domain. The trouble is somewhere within Thunderbird, thus why I have posted this issue on Mozilla Support.

Is the "another computer" on the same network? Your ISP may have a defective DNS server.