Hi Sanjay,
When you use Maintain order at run time option the messages will be processed in EOIO in adapter engine so if the file message failed to deliver then the next mail message will be in the queue until the predecessor message went into final status (successfully delivered).
Conclusion: If the file message failed the mail will not be triggered. if the file message successfully delivered then only the mail message will be delivered.
I hope i clarified your doubt. you can go ahead and implement my suggestion it will definitely work because i already implemented in my project and it is working.
Regards,
Praveen.