When you paste in eM Client, you get a tooltip option to choose whether you want to paste formatted, plain text or as a quote, so there is no need to copy as plain text. Just past as plain text instead.
If you mean copying from eM Client and pasting into another application, most offer a keyboard option to paste as plain text anyway. Like Ctrl + Shift + V.
Select any text within the email you are reading. It will be copied to the system clipboard; you save yourself a Ctrl+C, Cmd+C, right-click + Copy or whatever you’re normally doing. Without any formatting, the text is copied as plain text.
This function is available in many software programs that can be extended with add-ons. Such as: Thunderbird, Outlook, Gmail, Firefox, Chrome,…
It really doesn’t matter how or where you copy it from. When you go to paste it in somewhere, anywhere, just use Ctrl + Shift + V. It will paste as plain text. You don’t need any addons or utilities to do that, as it is built into Windows already. It should work in any application.