Ok, I now realize what you are doing. I was pasting directly from an Excel table (or list), but it's the same.
If I first paste to the Body of an email and then select a column and Copy/Paste to address fields the email addresses are pasted one under the other and separate when I click away.
Tried in many ways, in some cases had to insert an empty column on the right of the email column then select both and do Copy/Paste/Click away.