What kind of "list" are we talking about here? Thunderbird's own Mailing List, as set up in the Address Book, or something else?

If you do mean a Mailing List, select it in the Address Book and its contents will appear in the right hand pane, where you can sort the names, or email addresses, and so show them in alphabetical order. I'm guessing that's what "alphabetize" means?

What does "will not send a post to a list" mean? What happens? What doesn't happen? Are there any error messages?

By List I mean a list that is manly a subset of the Address Book. The Address Book entries can be arranged in alphabetical ordrer or reverse order by clicking at the top of the Book. Not so a list.

I make a List that contains copies of Address Book entries. I click Write and enter the name of the List after To. The Send and Send To buttons do not highlight. Th will not send posts to the list of addresses. I get an error message like Not in formof User@Host. But the addresses are in that format

Rebuild your Mailing List. This is an unfortunately common event with Thunderbird's Mailing Lists. :-(