Prevent TB downloading old emails to new account?

[portyforty]

I've recently begun to access my emails, firstly via my ISP's website, then through Thunderbird.
This enables me to notify the ISP of spam and trash unwanted mail, before I open Thunderbird to fully deal with the remaining mail.
It's just occurred to me that deleting all mail from the server in this way, would prevent Thunderbird from downloading old mail each time a user-account is copied or moved to another computer.
Am I correct or am I missing some point?

[DanRaisch]

How is the account set up in Thunderbird, as POP or IMAP? Find out using menu path Tools->Account Settings->Server Settings->Server Type at the top right of the dialogue.

[portyforty]

Mine is a POP, and yes, I know that this doesn't happen with IMAP accounts.
But I was asking, in part, because there seems to be a number of people with complaints about the issue of Thunderbird downloading old emails every time it's reinstalled or transferred to another computer.
So my question is mostly hypothetical.
Seems to me that, if POP account users deleted all their mail from their ISP's server, there'd be no deluge of old mail following a transfer.

[DanRaisch]

With a POP type account, if you delete the messages from the server inbox via the webmail page or through a setting in Thunderbird (unchecking "Leave messages on server"), there would be no messages for Thunderbird to download upon transfer or new computer set up. The deluge of old messages would come if the old messages were left on the server (not deleted) and an installation of Thunderbird set up on a new computer.