fix: Improve "Original Message" signature

[Myzel394]

Some email clients capitalize "Message" for example or add some extract space between the words and the dashes (ProtonMail does this for example).

This is what ProtonMail adds at the end

------- Original Message -------
(original message here)

[Myzel394]

#9 might be relevant to this

[toshsan]

Hi @dmfilipenko! Any chance this could be merged soon?

[baptistejamin]

Hey there!

Just merged a combined version of different PRs for this case under version v1.6.1