I don't understand. You can specify multiple recipients in the message. If you want them to think no other person received the message, you can use the MailMerge addon.

Looks like my problem self-resolved.

My question did not relate to the "TO" addresses of outgoing messages, it related to the specified "Reply-to" address to which REPLIES to those messages would be sent, instead of back to the original "From" address. In this case, I'd like replies to go to several people in addition to myself who share duties for an organization.

RFC 5322 allows for specifying a comma-separated list of one or more "Reply-to" addresses. When I tried to enter multiple addresses into the Reply-to field, I received an error message (that I was too dumb to record) when sending the message, and the message did not send. Similarly, when I tried to type in just the addresses separated by commas, since these addresses were in my address book, they were individually replaced by their normal address block displays and generated the same error message. This was with a TBird 128.(something)esr installation.

I just updated to 140.2.0esr and tried again. The message went through without generating an error message and arrived with the specified Reply-tos intact. So I don't know if 140esr does things differently, or if I just had a problem with my 128esr installation.