In send_message/6, check for any invalid addresses, and when present, send bounce message

[pheller]

Once send_message has optional flags to set bounce, hide_sender, etc., then it should be refactored to check that all recipients are valid USER IDs (can leave nicknames / mailing lists until later, since there is no means yet to manipulate these entries).

If any recipient is invalid, the message should not be sent. Instead, a bounce message should be crafted, where the message:

• has the original sender as the recipient
• has a from name of "PRODIGY SERVICE"
• has a subject of "Return to Sender"
• has the original message body
• has the bounce flag set
• has the hide_sender flag set
• has all original recipients as original_recipients