Who is the email provider?

Is the account set up for POP or IMAP?

The situation you describe could suggest that the Macbook uses POP and the others (Windows) use IMAP.

Thanks for prompt reply: you are correct; my Mac is on POP and my other two computers are IMAC. Can I change from IMAP to Pop and will that retreive my lost emails please?

Can I change from IMAP to Pop

and will that retreive my lost emails please?

The challenge is to get the messages from the Macbook back to where the other machines can see them.

https://support.mozilla.org/en-US/kb/switch-pop-imap-account

Personally, if using several computers to work an email account, as you are doing, I'd set them all up to use IMAP, having made sure that the email provider didn't have any silly policy about automatically removing messages over a certain age.

You could copy the account's file(s) from the Mac to the equivalent place(s) in the profiles on the Windows machines. Slightly more user-friendly might be to use the ImportExportTools add-on to export and import your messages.

https://www.howtogeek.com/99423/email-whats-the-difference-in-pop3-imap-and-exchange/

But you do need to see if it is your email provider deleting these messages; there may be no point switching to IMAP if they keep on doing this.

If you were to set up the account again on the Mac but using IMAP, you could then move messages into it and they would then appear on the Windows computers. But this isn't nice if you have hundreds of messages.

Thanks for your help. I am going to have to manually forward email receipts from the Mac computer (which is POP) to my other computer and print them out as that is the main aim for my accounts. What I dont understand is why two of my computers started deleting emails because they didn't before.