Basically I had the same problem, and if you had an iphone spits on ground I would say yes (you can set more than one smtp server up and if it fails on one it'll try on the other) . . you can just get round it by turning off the phones wifi when you want to send an email (at home)
Emails can be sent over the phone network using their smtp server (smtp.orange.net for example) but as soon as you hit your home network (via wifi) the orange out-going server will not work (because it doesn't know who you are because you are going through your own ISP), where as if you change your smtp server back to ntlworld it would work (because you are using your home connection)(hope that made sence?)
O2 solved this issue, because they had a user name and password for their smtp server, so even if you sent an email over wifi (at home) it could authenticate who you were and subsequently send it.
I tried explaining this to orange . . . it fell on deaf ears