They're not appropriate for formal communication. In those cases, a written warning that you mean a certain passage to be read a certain way will help. Not giving in to the impulse to be sarcastic, ironic, etc., is even better. When you send digital images attached to an e-mail, remember that large graphics files can be corrupted, in which case they won't display. Large files can quickly fill and overload someone else's mail quota.
Asterisks act as italics.
Proofread and correct your spellings before you send the note. When your notes are rife with misspellings, at best you appear sloppy and careless, at worst uneducated. When Writing Keep your messages short and to the point. When replying, don't automatically include the original message unless you have good reason. If you're replying to specific parts of a message, snip out the extraneous material.