re : However, I sent an email with attachment to my GMAIL address, which is not synced with Thunderbird,

I presume you sent the email using Thunderbird. You sent TO a gmail email address, but that gmail account is not a mail account in Thunderbird. Did the email explicitly show your email address in a Cc field ? Do you see a copy of that sent email in the 'Sent' folder and does it have your email address in the Cc field? But what account sent the email ? What comes after the @ in your email address? Is it another gmail address ? Did you put the same email address which you used to send an email - it was in the FROM field - you put same email address into the Cc field ?

Some mail services will suppress CC to oneself. gmail is one of those servers. If you are sending using a gmail account then gmail will not honour the Cc to yourself. In gmail webmail account try this to see if it works. Settings->Filters (in the Gmail web interface), make a filter for all message FROM you, and use the "never send to spam" option. That should force a copy to your Inbox.

Jeezz! I worded this question carefully so I would get more questions in return like this. In the cc: field I used the same address I sent the message to GMAIL from, the one already synced with Thunderbird. My God, if I used the GMAIL address in that cc: field, then that would have meant possibly another lost email message. This is a new problem that only occurred as of last month, never occurring before, my not having changed my email sending habits.

re : In the cc: field I used the same address I sent the message to GMAIL from, the one already synced with Thunderbird.

I know that because you have already told the forum.

What I do not know is whether the FROM which you used was another gmail address thus using the gmail smtp server and if it wasn't then what smtp server did you use to send? Hence the additional questions.

As I mentioned there are some servers that do not allow Cc to same address as used for sending, but at this point no one can check it out because you've not told us what smtp server was used.

I also asked: does the copied 'Sent' message in 'Sent' folder correctly display the Cc? But you have not told us the answer. That would confirm the message was sent correctly by Thunderbird to the smtp server. In other words, if message was sent correctly to server then the reason the Cc was not actioned upon is not Thunderbird, it's the server - but then what is the smtp server?

Sometimes it can be calculated what the smtp server is by looking at whatever is after the @ in the FRom email address or what is the 'server name' of the outgoing server smtp used for that From email address. Hence the request for additional info which you have not as yet supplied.

"What I do not know is whether the FROM which you used was another gmail address thus using the gmail smtp server and if it wasn't then what smtp server did you use to send?" - No. I wanted to italicize that quote; but the editing buttons above this box don't work, and my keyboard shortcuts for editing don't either. I am on my Windows 10. Thunderbird is on my Mac. I will get to the rest when I am on it again. Here it is the next morning and, now back on the Monterey OS, I confirmed that I used my Comcast email address in the cc: field. Later today I will telephone my ISP technical support about this abnormality of the last two months. There is no other address synced with my Thunderbird on this system. Thank God; otherwise I would have trouble with that address too! Looking at your question in more detail, the From address is the same as the cc: field. I am suspicious because someone might have tried to send messages to me who does that on a regular basis. Those messages may have bounced. The Get Message command does not seem to do anything. In the Sent folder the cc: and the From fields show the Comcast address. Comcast addresses end with @comcast.net while GMAIL ends with @gmail.com. The Outgoing Server in Thunderbird under Account Settings is smtp.comcast.net and the Port is 465. I use POP Mail Server mail.comcast.net with Port 995. I hope there is enough information here to solve what's happening.

comcast - that's xfinity and you are using Pop. This means you would only be able to download emails that have been put into the server Inbox folder.

If the server is putting any emails you send to yourself and Cc = your email address into 'Spam' folder then you would not be aware of anything in thunderbird because the POP account has no knowledge of the server Spam folder.

I believe xfinity also empty spam folder every 7 days. https://www.xfinity.com/support/articles/set-email-deletion-settings

Logon to webmail account and check out what is currently in the server Spam folder and what is listed as spam email addresses - maybe your email address has got into that list. Maybe you need to set your email address as a whitelist address.

Someone else had a similar issue, but they were accessing emails via laptop and phone and discovered it was their phone messing things up. https://forums.xfinity.com/conversations/email/email-from-myself-to-myself-going-to-spam-on-my-laptop/642aed324d04032f94614581

I am on my Windows 10 now. Yesterday a Comcast technician helped a little with this problem although my eFax is still not fixed. He explained that I had an address on my Safe List. Addresses there mean that you only receive from those addresses. Later I will get on the the Mac again and look further into this. Now, I just realized that I should have written Monterey instead of Catalina. Earlier I had to reinstall Catalina and upgrade to Monterey to make available video functions. Now, I am on the Mac Monterey (not Catalina as indicated in the beginning). For one thing, my Thunderbird did not come with a Spam folder; so I created one called Junk. I do not know if that causes conflicts. On my smartphones I only access my GMAIL, not my Comcast one. In my Comcast account in the advanced email settings, I do not have use email safe list checked. What is there is the email address my telephone provider requires for eFaxing. So, as the technician explained yesterday, having that checked meant that I could send only to that address and expect a response, such as a cc: copy. Unchecking that box made a difference because I now receive those copies and other messages that I regularly received. Thank you.

I do not have a detailed explanation; but today I noticed that I am getting the copies. Thank you. Hopefully we will not have this problem again.