1. Wrong Site
The most common reason for an email message failing is that the site part of the address (the bit to the right of the "@" sign) is incorrect. You may simply have mis-typed it, have transcribed it wrongly from wherever you saw it, been given it incorrectly in the first place, or the site itself may no longer be a valid address. For whatever reason, the system will return your mailing with an error message something like:
This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: email@example.com Unrouteable address
If you can spot immediately where the problem lies (in the example, a clear mis-typing of "physics"), simply resend the message using the correct address. The actual returned message will contain your original text, and most email programs allow you to re-send a message, editing it to remove any of the unwanted error messages and header information. If the failure message indicates that you did actually type what you expected, then you probably have the wrong email address. Go back to your original source.