Have you got a complete unicode compliant font installed on your machine?  (Most non-ancient computers do now.)  Some individuals still use non-Unicode fonts, which will not display properly if you do not have the same installed.  Does it always happen?  Example?

But it may also be your mail interface.  My Outlook mailer, for example, will replace every macron-a (ā) with a question mark — only that letter, not ī, ū, etc. — unless I include an underdot (or overdot) character in the message as well.  For this reason, I include one in my signature, in case I forget.