We have had several problems with Outlook Mobile. Microsoft updates the app twice a week on average.
About a month ago, users would SEND, and Outlook Mobile would show it SENT, yet the app never touched the SmarterMail server at all. This was fixed by Microsoft with yet another app update.
More information is needed.
Can you determine whether this is a SmarterMail issue or Outlook Mobile issue?
Does the e-mail subject and recipient show-up fine in webmail?
Can you produce the issue with 2 recipients, or 3 or more?