Please consider creating examples of what you are doing, eg. sent to recipient A, copying persons B and C, using account X. I do not understand what you mean by having the mails appear in other accounts.

Thank you, David.

In Thunderbird, I have accounts X, Y and Z.

The "primary" account (let's call it, for lack of a better word; I don't understand why one account works, and not the rest), X, displays all of the CC/BCC recipients when the sent mail is checked.

Accounts Y and Z display only the recipient from the "To" field.

If there are only two or three recipients who are CC'ed or BCC'ed, let's say A, B and C, sometimes these are displayed upon checking the sent mail of accounts Y and Z. (Sometimes. Not always.)

Yet if there are more than two or three CC'ed or BCC'ed, let's say A, B, C, D, E, F, G and H, these CC/BCC recipients are not listed when the sent mail is checked in accounts Y and Z.

Based on past experimentation, A, B, C, D, E, F, G and H are not only not displayed in accounts Y and Z, but the email that was sent was not received by them. As I wrote, I accidentally learned from a recent attempt (I used one of accounts Y and Z for a bulk mailing because I had forgotten about this idiosyncrasy) that A, B, C, D, E, F, G and H did receive the email (because one was returned), although A, B, C, D, E, F, G and H are not displayed in "Sent."

You, and possibly others, are most kind to be providing your attention, and it is appreciated.

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Thank you. As I understand it, when you send to A, B, and C from Accountone, everything is proper. When you send email to them from Accounttwo, sometimes it works properly and sometimes not. I suggest you post this as a bug at https://bugzilla.mozilla.org/home but you may need to be specific in identifying the account domains and that bulk mailing is involved. I emphasize the importance of providing extensive detail, as what you are describing seems quite unusual. Thunderbird treats accounts individually and the fact that an account acts this way requires in-depth analysis to understand what is happening. Good luck to you. This issue far exceeds my ability to help.

My gratitude to you for expending your valuable time and brain power, David.

As I'm still hoping for a solution (reporting a bug is something I probably won't get around to), perhaps another or others might have an idea about why this problem is occurring. Thank you.

I am making assumptions.

If your account is IMAP (this does not apply to pop sent folders) then your providers policy of BCC retention applies. Google remove all BCC entries from send mail. That is there policy. So mail sent via Google servers will not have BCC information stored in the IMAP account. ( I understand they are not the only one, but they are the one I am familiar with) SO that covers the BCC scenario, but I am not aware of any provider that removes CC information as it is not considered "Blind"

Based on past experimentation, A, B, C, D, E, F, G and H are not only not displayed in accounts Y and Z, but the email that was sent was not received by them.

That would be consistent with the mail never being sent to those addresses. What make you so sure they were included?

In addition, in the past, experimentation demonstrated that the mails did not get sent to the additional CC/BCCrecipients, although I was surprised to accidentally discover today that the emails did get delivered.

So I am perhaps confused.

Pick one of these mails with "missing" entries in the CC and right click it and select edit as new message from the menu. Are the details missing from the new message that opens?