Mail App Question

Hi,

I have a BT email account set up on my mail app on my MacBook Pro, and I have also set up a new iCloud email address on there too. My iCloud email will be my main one from now on as I phase out the BT one. The BT one has lots of folders in it with various emails I need to keep, I have managed to add new folders under the iCloud email address but drop the BT ones in there instead of having to forward them all to my new email address. They seem to be in there ok and also seem to be on iCloud when I login via a browser. I have temporarily removed the BT account from the Mail app to test if the emails remain, and they seem too. What I was wondering is if I delete the BT email address entirely I'm assuming this will remove those emails, even though they are now in the iCloud folders?! Has anyone ever done this before?!

I'm also wondering if I delete the old (and now empty) folders from the BT mail whether I will loose any emails or are they safe in their new folders?!

Many thanks for your help.