Out of the blue my iPhone mail app kept either crashing or simply exiting back to the home screen 2-3 seconds after starting. Unable therefore to read or send mail I logged in via my MacBook to my Mobile Me account assuming some errant e-mail was causing problems.
I deleted some spam and some older messages and tried again but still no luck. I then clicked on the trash folder and selected Empty Folder from the drop down action menu. This did the trick and everything now appears to be fine.