If you are setting up Thunderbird from scratch. Using IMAP or POP the server will send all mail it has stored. If you are using Gmail there are setting on their web site to make only mail from a certain date download in a POP account.

Once the download starts particularly with IMAP there is nothing to disable.  The account is synchronizing to it's canonical source (the server).  That is how IMAP woprks.

Hi Matt, many thanks for your reply.

I am not newly loading Thunderbird. This problem has arisen before and seemed to be solved by restarting the laptop in Safe Mode. But it has returned and now Safe Mode makes no difference.

When setting up, the installation of Thunderbird stalled for literally hours when trying to use IMAP, but instantly progressed when I switched to POP.

Is there any other advice you can give regarding the POP option?

With thanks again and best wishes, Sandra

How can I stop Thunderbird from re-downloading all emails on a MacBook Air? I have disabled in Server Settings the automatic checking and downloading of emails. I am using POP rather than IMAP - I could not complete the set up of Thunderbird using IMAP. Thank you for any help you can provide.

I have the same problem, but I have set IMAP not to automatically download to avoid the slowdown of Thunderbird. I only go to the IMAP INbox once in a while, have been using several POP accounts for decades and never had such problems there. I think IMAP is not designed to have more than 1000 mails in a single folder, unless you have a really fast internet connection. So I think the general advice to "switch to IMAP because it is more modern" is probably a bad advice if you are hoarding emails like I do.

The other problem is that if your mail server every closes down IMAP access (like it happened with me when godaddy switched to Exchange only for a business mail address) you can actually lose access to the mail you think you have already downloaded (if it's in the Inbox). That's really an unacceptable risk, so POP is a lot safer in that regard. At least you can organize your own backup schedule and store your own emails on your own devices (or encrypt it if you insist to back up to the cloud). IMAP just takes away too much control.