MacBook emails - can someone help with a technical question?

[HFJohnson]

We replaced our old Mac laptop recently with a new one and I've discovered that I seem to be losing older emails. After a lot of googling I think it's because when I delete emails off my phone, they also get deleted off the Mac, and vice versa, which never happened before.

Does anyone know how I can stop this? I like to treat the two separately as I might want to keep some emails on my phone and others on the laptop for various emails.

It also seems to be affecting my bt email inbox aswell as emails are also being deleted from here too. Again I don't really want this to happen.

Finally, because of the above I seem to have lost an email from a month or so ago and I can't find it any of my inboxes or trash accounts - any ideas how I could find it?

This is frustrating the heck out of my so any advice is very welcome!

Thanks!