"You have a Windows Mobile 6.1-based device that is configured to use a POP3 e-mail account or an IMAP e-mail account. On this device, you experience the following symptoms:.
You unexpectedly lose the ability to send e-mail messages
E-mail messages remain in the Outbox folder after you synchronize the e-mail account.
You may receive the following error message when you try to send an e-mail message:
The message cannot be sent
Even though you lose the ability to send e-mail messages, you continue to receive incoming e-mail messages."
What I find interesting, and frustrating, is this is because Microsoft has given yet more control to the carrier. As you read the rest of the KB article, you get to this paragraph:
"This behavior is associated with a feature that is introduced in Windows Mobile 6.1. The feature allows for mobile operators to specify an alternate SMTP server name that is used if e-mail messages cannot be sent by using the user-specified SMTP server name. If the mobile operator does not specify an alternate SMTP server name and if the Windows Mobile 6.1-based device does not connect, the e-mail account is corrupted and cannot send e-mail messages." Read more...