Symbols for other languages

I usually write in two languages. Spanish and English. I don’t need the spell checker, but I used to be able to write accents and symbols like ¿ ¡  by using the alt # codes. Now I can’t do it, and my emails look full of strange code when the recipient gets them, though they look normal to me.  Why is that happening?

Hello Rebeca,
is the message corrupted already in your sent folder or only when it arrives to the recipient? Dos the same happen when you send a message to yourself? What mail account does this issue happen on?