text encoding on Apple Mail (version 10.3)

HI I am having a seemingly old problem with Mail 10.3 : occasionally (not sure when) I see in messages that I have sent out and gotten back as a reply, my accents converted into the cryptic encoding symbols.

I read on the internet that this could have something to do with the text encoding version of my mail program (not with the option Rich Text which I always use by default and which works generally well).

However, in my mail version there is no menu entry "Encoding" under "Message".

Can anybody say something relevant to this problem?

Thanks in advance,

HM