cms₂EM Outlook-winmail.dat
For Microsoft's Outlook (all versions)

The character encoding choice will depend on what language you use most often; different encodings will give you different character repertoires. The default setting is good for most Western European languages, but other settings might be needed for languages using different characters.

If you go back to the main Options screen, and pick the Spelling tab, a button there will take you to the Auto-Correct page:

There are a few items there in the default setup that might put nonstandard stuff in your messages. It's a good idea to remove the autocorrect items involving the "TM" sign, curly quotes, dashes, and the ellipsis (...). These will "autocorrect" the plain-ASCII versions of these things to characters from the proprietary Windows character set, which, as I discuss in my character sets article, are not part of the standard ISO-8859-1 character set. There are standardized ways to insert them, but I don't particularly trust any program from Microsoft to use them, and support can be problematic anyway; plain ASCII is safer.