SOURCE - Each Correspondent. I group all (both snail mail and email like this:
Correspondent - John Smith
Correspondent - Bob Jones
By beginning all of these sources with the same word groups them all together in the Source List.
Then, each separate letter or email goes into the citation detail. You could put the email address into the Source somewhere. I started with putting it in citation detail so I am using that going firstname.lastname@example.org
a subsequent email from this correspondent is another citation and this is Citation Detail:email@example.com
If I am entering the data into FTM, why attach the email as media? You've already got the data and the source.
However, some emails are narratives and then I usually just copy it into "Research notes:", or if it belongs to a Fact, I paste it into the Fact Note for the fact.
In reading your post, I'm not sure if you understand the difference between Paste Duplicate Citation and Paste Link to Citation.
Paste Duplicate is a separate citation for a subdivison of the source, in this case, a specific email on a specific day.
Paste Link is a clone of the copied citation.
For example, if you have a list of 30 soldiers on a page of a book, you can enter 30 soldiers with separate citations by Paste Duplicate Citation, or attach only ONE citation to 30 different Facts/People by Paste Link. You chose to Paste Duplicate or Link at the little down arrow after you have copied the citation.