There is no difference between OS.

However, there is a difference between POP and IMAP.

POP will have an option to 'Autmatically download new messages'

IMAP will not have that option for very obvious reasons. Imap mail accounts see a remote view of folders on the server, so they synchronise anyway.

It sounds more like you were viewing a POP mail account in Vista and Win 7, but viewing an IMAP mail account in Win10.

Do I need to change to a POP server to choose when to download? Will I lose any email functionality?

Pop mail accounts can only access the server Inbox and download any emails not previously downloaded. So if you have any emails in any other imap folder (not Inbox) then those emails will stay on the server and not get downloaded. If you wanted to download them you would need to do one of the following:

• log on to webmail and move emails from eg: server'Sent' folder into server 'Inbox' and then in Thunderbird, click on 'GetMessages' to download. Then you would need to move them into the pop 'Sent' folder.
• Using the imap mail account: synchronise folders for Offline use to download a copy into Thunderbird profile folder, then in Offline mode, right click on highlighted/selected emails and select 'Copy to' and select a suitable folder in 'Local Folders' mail account. Check you can read those copied emails.

Pop mail accounts are best used if you only access mail from one computer. They can be set to leave a copy on the server, so you could also see received emails by webmail if required. Emails are stored on your computer in your profile folder. You can read them even in Offline mode. When you delete an email, you can choose whether to only delete the one in Thunderbird or if a copy is on the server, to delete both.

Imap mail accounts keep emails on the server, so making it easy to see all sent and received emails from more than one device. Usually only headers are downloaded and the actual email contents are only downloaded when selected. They are stored in a temporary cache to facilitate viewing. You can synchronise folders for offline use to get a copy into Thunderbird profile folders to allow viewing in Offline mode, but these folders will auto synchronise with server when going back into Online mode to update with server. Anything you delete in Thunderbird will get deleted off the server as they are one in the same folders.

So, to answer: 'How do you stop Getting mail automatically when using an Imap mail account?' Synchronise folders for offline use to get a current copy into Thunderbird, then go in 'Offline' mode to stop all synchronising. When you want to 'Get Messages', all you need to do is go back into 'Online' mode and synchronisation should be automatic. However, in 'Offline ' mode you cannot send emails., but you can write them and choose 'Send Later' to put them in your Outbox, so when you go into Online mode, you can send those emails.