Thunderbird offers at least two ways to go about this; a Mailing List and the Mall Merge add-on. However neither of them handles failure very well, in my limited experience, and typically if the mailshot fails, it's hard to know who was successful and who needs to be done again.

I'd be inclined to look elsewhere. The Libre Office suite has a mail merge that uses its own smtp client, and for that many addressees I'd also look at MailChimp.

Thanks Zenos I will look around for another source. Compass1

Perhaps someone could enlighten me how Outlook handles failed sends better than Thunderbird?

Having said that no one should be sending email to groups of more than about a dozen people these days. Private mail providers Google, Yahoo, Outlook, AOL and most ISP's will simply refuse to deliver mails in large volumes. They all have differing daily limits, but my guess would be 400 is probably over the daily limit of most providers.

If you want to do mailing list type things, then a professional mailing list company is the way to actually get your mail delivered and not have your IP address blacklisted for sending SPAM. Groups like MailChimp offer free accounts for the sort of mails you are talking about and offer all sorts of enhancements on normal mail with regard to deliverability of bulk postings.