Microsoft as officially reported a bug on Windows Mobile 6.1 that makes you unexpectedly lose the ability to send e-mail messages.
I had personally gone through this experience and had to make a hard-reset on my i780 to solve this.
What happens is you are not able to send out any e-mail messages on one or more e-mail accounts.
There is however a simpler workaround for this issue reported here.
Here it is:
To work around this behavior and to restore full e-mail functionality, you must delete the existing e-mail account and then set up the e-mail account again.
Important All the e-mail messages in the Outbox folder on the Windows Mobile 6.1-based device are erased when you delete the existing e-mail account. You must re-create these e-mail messages after you set up the e-mail account again.
Note This workaround is only a temporary solution. Until a resolution becomes available, this behavior may reoccur for the new e-mail account.
Though not the ideal solution, it works, so lets hope for a ROM upgrade on the near future that might solve this issue.