Thousands of e-mails

[#1]

I use Thunderbird and on opening just now, the message I get is that it is downloading over 9000 e-mails dating back 5 years!
Can anyone explain what is happening and what I should do?
Thanks.

[#2]

Do you use POP3 and do you have it set to permanently leave the messages on the server?
If so this is an unfortunate consequence of the mail difficulties over this weekend.
It is worth noting that if you ever had a problem with Thunderbird such that it forgot which messages it had already downloaded (not an unknown problem) you would see exactly the same thing.
You may be able to clear them out using webmail (but you will struggle with that many in your mailbox).
If you interrupt it, the next time you start it up it is probable it will start from the beginning again.

[#3]

....or set up an IMAP connection.

[#4]

Yes, I do use POP3 and I believe that Thunderbird is configured to save messages. I will change this setting asap.
Looks like i've got  a big housekeeping job to do.
Thanks for your guidance.