Contacted my ISP (Media Temple) and the problem does not seem to be on their end

is a send fails, it fails.

A better answer would be to use an address book, not a list. Why? because then you do not have this problem, can filter mail based on the member of that address book. You can not filter on the members of a mailing list.

The draw backs. Typing the list name does not work for addressing

Using the contact pane in the compose windows you click the first entry, hold shift and click the last entry and then click the relevant button below.

Alternatively use the mail merge add-on and send personalized emails to the list. Much nicer.

Thanks for the Mail Merge suggestion. That is indeed much more elegant.

Your response ... use an address book, not a list... and the related suggestions confuse me. TB Address Book allows one to create a New List and creating and then selecting that list (in bcc in most cases) has worked most times (and it is way simpler than finding the desired addresses from the contact pane each time).

In this case, the design flaw seems to be that one unresolvable address led TB to abort sending to all recipients... So, yes, ...if a send fails, it fails..., but I was wondering about the why...

spmcc123 said

In this case, the design flaw seems to be that one unresolvable address led TB to abort sending to all recipients... So, yes, ...if a send fails, it fails..., but I was wondering about the why...

It is in the nature of SMTP servers that send mail. The mail client gives it the list of email addresses, it says no one is dead and closes the connection send aborted is the short summary.

Email works in a very defined space and Thunderbird can not do what the server refuses to do. That sending framework is refered to as an RFC, and the one for SMTP is to be found here. https://tools.ietf.org/html/rfc2821