Importing emails from talk21

I have just opened my first zoho email account.

I have used talk21 (a BT Yahoo! service) since the very beginning of email and have 18 years of lovingly filed emails. These will all be lost in around two weeks time as talk21 will no longer be supported.

I urgently need to import a large number of emails from my talk21 account to my zoho account. Can you send instructions on how to do this, please?