Outbound Mail is working fine until you tick the "Outbound E-mails to use Inbound Mail box" checkbox in Outbound Mail Server Settings
..but mails are not being sent. In the Application Event Log there is the following error: 'The specified string is not in the form required for an e-mail address.'
This can be cause by incorrectly mapping a Message Attribute to the "Reply E-mail Address" Business Attribute in the Mail Mappings:
The Reply E-mail Address and Reply Display Name attributes are used by Outbound Mail to determine what name and email address to send mail under when "Outbound E-mails to use Inbound Mail box" is ticked, so using it in a mapping causes a conflict, resulting in the above error.
To fix, simply remove the mapping.