POP and IMAP are email standard protocols. There is lots of posted info on how email works. Google POP vs IMAP. What you want to do is standard operation for email account setup as IMAP. As for how this relates to Thunderbird, read this.

https://support.mozilla.org/en-US/kb/switch-pop-imap-account

Tags are provider dependent. Some sync tags on their network, some do not.

I changed my settings to match the above. It didn't seem to make any difference. There is no synchronization among my 3 computers.

Ah - you "changed" your settings? Which ones?

It isn't possible to change an existing Thunderbird account from POP to IMAP or vice versa. (I am not aware of any email client that allows you to do this!) You set up a new account, using the alternative protocol.

Identify any messages in the old account that you want to keep, and move them to the new account. You can then remove the old account.

When you say "I changed my settings" I hope you mean you disabled your POP setup in Thunderbird and recreated the account as IMAP.

My setting already was imap. It never was POP.

So your providers account and all 3 computers are set for IMAP? Then what exactly did you change?

Go to the Help Menu and click Troubleshooting Information. Click Copy to Clipboard. Paste that info into a reply window here on the forum.

Hi holmsale, when you use IMAP to connect Thunderbird or another email client to Gmail, then moving a message to Trash on either side -- mail client program or Gmail webmail -- should synchronize to the other. When multiple IMAP clients are connected, they should similarly receive changes from Gmail at regular intervals.

Is that not working in any direction?

I have 1 gmail account & 1 yahoo account. I have not checked either in settings to see how they operate I have 4 AOL accounts & I also have not checked to see how they operate. What I have done is to go to THUNDERBIRD settings & set them to IMAP. That did not change anything for my AOL accounts. I rarely use gmail or yahoo so I can't determine if those messages are in sync. I do not think I can change AOL settings inside of AOL (meaning imap vs POP). When I was told to change the settings, I assumed I was being told to change them in Thunderbird.

I just replied to someone else on this. Thanks. Most of my accounts are AOL. I do not think that I have the power INSIDE OF AOL to change settings. I only changed them in Thunderbird. I also have 1 yahoo & 1 gmail account. I rarely use them so don't know if they are in sync.

You said that it is not possible to change an existing Thunderbird account. You said that I must delete it & open it as imap. This is my reply to that: When told to change my settings, I found most already imap but I did find a POP. I changed it & it changed. Maybe it is confirming what you wrote but that didn't make anything go in sync.

As I said above, I can't change AOL to POP or imap. Or I don't know that I can.

Then I suggest you ask your email provider how to use their system. IMAP is required on all devices/computers and for your account on the providers server to sync. https://help.aol.com/articles/how-do-i-use-other-email-applications-to-send-and-receive-my-aol-mail

If you chose to ignore the warnings not to change an existing account inside Thunderbird from one protocol to another please do not complain later that you lost all your data.