If you emailed someone and got an automated "bounce" email (e.g. "Undeliverable"):

1) Read the description for hints to the problem: 
    E.g. "The e-mail account does not exist at the organization this message was sent to."

    or "Message was blocked by content filter as spam."

2) Confirm that you Sent To the correct email address.  Misspellings matter.
3) Check that you had good descriptive text in your email's Subject and Body.  If you don't, recipients' filters will often treat it as spam and block it.

4) Make sure any Attachments are not .EXE or other executable file types.  Often, changing the extension to something like ".DAT" will let it pass (of course, be sure to tell the recipient you did so they know how to handle it).

5) If all else fails, Forward us the bounce message AS AN ATTACHMENT (instructions at that link) and we will run diagnostics on it to troubleshoot the problem.