Three (3) dots below an open email? What do they accomplish?

That is a function of the conversation feature in eM Client. It indicates that there is quoted text in the message that comes from a previous reply, but has been collapsed/hidden to simplify the view. To view the text, click on the dots. To disable the feature, go to Menu > View > Conversations > Disable Conversations.

Thank you very much for your reply. Very clear.