POSSIBLE SOLUTION!
I have had this problem too since I upgraded to an iPhone 5 (my mail goes through Telus). Tried all the suggestions in this thread, nothing worked, although after switching my iPhone off completely, then back on again, the mail 'disappearing' was resolved and the mail not sent was placed in my 'Outbox', so at least I knew what hadn't been sent.
Well, I have access to my husbands and my sons mail accounts from my iPhone too, so I thought I would send mail using both of their accounts just to see if the same thing happened. My husbands e-mail to me at my work e-mail address failed and was placed in the 'Outbox', but my sons worked and I received it to my work e-mail address. So my thoughts then were "Well, what is different on the settings on my sons mail account that means it will send e-mail without a problem?". The only difference was that all the 'Other SMTP Servers' were set to 'Off' and the the Outgoing Mail Server - Server Port was set to 1025, whereas mine and my husbands was set to 587. I changed them both to 1025 and for now, fingers cross, touch wood, rub my lucky rabbit tail, it works. Mail is sending and being received without a problem. Yay! not sure if these Server Port numbers are unique to Telus or not, but it may be worth trying..... If it fails again, I will report back here