- Use any character and all punctuation freely in email.
Every character is save (as long as you remain within the boundaries of Unicode, at least).
If anything you write is displayed as gibberish at the recipient's end, something is broken in between. Most likely it is the recipient's email program that is not displaying certain characters right, or an email server in between introduces translation errors, or maybe even your email program does not encode the message in a standards-compliant way.
Whatever it is, it should not stop you to use these characters. Instead, try to find out whether both your and the recipient's email clients support MIME — they really should —, and if they do, ask your ISP as well as the recipient's ISP what may cause the problems and ask them to implement fixes.
Email should really support all languages of the world, and only if errors are uncovered they can be fixed. Working around them is certainly useful when it's necessary, but it's not the right thing to do, I think.
Only if nothing helps or if, for some reason, you must be sure to use only the least common denominator of all email systems, you can restrict your writing to the characters absolutely safe to use in plain text email.