I assume you mean a mailing list these do not show you who is in the list of the composer, but the recipients all get the list unless you send the email to BCC instead of to or CC

Undisclosed recipients is generally what Google changes the BCC fields of outgoing email in your sent folder to. Apparently the sender is not supposed to know who they sent mail to.

So you sent a message to a mailing list and some of them did not get it. That does not mean they were dropped. In some cases the message is probably in their spam folder. Others may had opted to only receive email from people in their address book. A poor decision to be sure, but one a number of providers offer.

So I guess we need to start with how do you know they were dropped (leaving out that non delivery is not evidence they were dropped)

The answer from Matt at Mozilla Support Forum is not addressing the problem I am facing. Here are the facts of the problem:

1. I created 12 lists of e-mail addresses in the section 'personal Address Book'
2. I entered individual 2-56 recipient e-mail addresses in the lists, and verified that all were correct (no typos)
3. I sent the 12 lists in 12 separate e-mails, each sent via Bcc with lists named undisclosed-recipients-x or -xx to protect the privacy of some recipients; I was fully aware that this would result in no individual recipient's automatic confirmation that the e-mail was sent.
4. In subsequent phone or e-mail conversations with some of the "recipients" (from different lists) I learned that some of them did NOT receive the e-mail. Those included my own daughter, with whom I talk every day; however, her husband (who was on the same list) DID receive the e-mail.
5. The only logical explanation I can think of is that some of the recipients on my lists were DROPPED. I would like to know WHICH RECIPIENT'S ADDRESSES WERE DROPPED and WHY.

I thank you in advance for addressing the right problem in your answer.

I answered your question, but because you can not think of another logical reason, in your apparently limited experience of such things. You are of the opinion that Thunderbird thunderbird dropped some of the recipients and you want to know why.

So I will put it this way.

Email comes with no guarantee of delivery. Just a snail mail before it came with no guarantees. If there is a problem with sending mail you are notified at the time you press send, Thunderbird holds an open connection between the mail server and itself and only stops displaying the message being sent when the transmission is complete. If the peoples addresses were in the send part of the email then they were sent or you got an error. You say nothing about an error so I must assume they sent without one. Certainly if the addresses had typos that made them invalid they would have been flagged at that time by your provider.

Once an email leaves Thunderbird delivery is up to your mail providers and the general connectivity of the internet. Thunderbird knows nothing and get nothing back in the normal process to indicate were the mail goes or what happens to it. Just list when you drop a letter into a street box. You know nothing more of it. it gets delivered mostly, but if it does not, that is unfortunate. Email is exactly the same.

So to address your number 5. No one will have a clue why your mail was not delivered to the relevant recipients but as I said the first time around failure of delivery is not necessarily a failure to send. It is very very unlikely the failure of delivery has anything to do with Thunderbird. There are millions of users and I have not heard of anyone else using mailing lists that has had more than the normal delivery problems. You are the only one here at the moment I am aware of saying mail was dropped.

The content of your email, the number of undisclosed recipients the presence or absence of a To: in the email are all deciding factors various mail providers use to decide if a mail will be delivered even after they have it.

Do you daughter and son-in-law actually share the same email address? I have dealt with a lot of questions here over the years where folk are emailing like as follows. Jill <owensfamily@somedomain.net> and Bruce<owensfamily@somedomain.net> and having issues with only one mail being delivered, but they are actually to the same email address and hidden or not delivered by the provider as duplicates. Only one gets through.

The bottom line is your mail provider sent the mail or not. Only they can tell you from their SMTP logs if the emails went out and if not why not. So you are asking the wrong people really. But if you ring you provider they will ask if you can send from their web interface and if you can claim email works. So I suggest you simply accept that because you click send it is not a guarantee of delivery.

Mentioning our daughter and son-in-law. Do they share an email address?