I admit to knowing little to nothing about POP, but is it possible that the two PCs are set up for POP access? I ask this because that doesn't happen with IMAP.

Yes, they set up for POP access, but it was done automatically by default and is out of my control (maybe because it is a Gmail accpunt).

If one of the devices have accounts for POP3 protocol and is used to receive or send emails, this means that the other device can see only the sent emails on the server and not the already downloaded ones. This means that if you want to see all emails on any device, then all devices must have accounts under IMAP protocol. This is the philosophy behind IMAP or POP3 protocols. Choose the one suits you best.

Generally, TB defaults to IMAP. That would be my recommendation. You cannot change from POP to IMAP; instead, you must delete the account and create new.

Unfortunately it is above my pay grade, if you see what I mean. :) OK, let me try.

click tools>accountsettings, highlight the account, and scroll down below account settings and there is a dropdown menu for 'Account Actions', one of which is to delete the account. Then you can start over and set up the account as you did earlier, but as IMAP.

David is right. IMAP or POP.

Note: It is not obligatory all accounts to be IMAP or POP. You can have one account under IMAP and the 2nd different one under IMAP. TB asks you to choose the communication protocol on account creation.

Thanks David and Lampros, So if I understand correctly, on one machine the account may be IMAP and on the other POP? If it works, then soon I will start bugging you with another question: how to migrate the stuff (emails, folders, address book) from one machine to another. :) There are many instructions for this online, but all are different and most pretty outdated.

OMG. Account on 1st device POP Same account on other device IMAP.

If you want consistency, you will make both of them IMAP.

I changed it on the new machine from POP to IMAP and it seems that now it works (thanks again !). As to the old machine, what will happen with local folders and address book when I delete the account?

David, it is not a matter of consistency. If a user is wants to follow up all emails from many devices i.e. mobile, desktop, laptop, then it is obligatory all accounts to be created under IMAP on all devices.

@lampros, what you just wrote is an indirect definition of 'consistency'.

Using import export tools ng, you can export your emails and then to import. Do this task before deleting the account.

We have to explain things in order some users to realize what they are doing. Many problems mentioned in this forum are caused because of the negligence of the users to learn basic but important things before they use Thunderbird. If our friend samuel knew the difference between IMAP and POP protocols, he would face no problems.