This behavior is associated with a feature that was introduced in Windows Mobile 6.1. It provides a way for users to send e-mail through an alternate outbound e-mail server provided by their mobile operator in the event that the default outbound e-mail server becomes inaccessible.
The problem occurs if a mobile operator doesn't use this feature, and therefore doesn't supply a value for the alternate SMTP address. When the mobile device fails on any attempt to connect to the e-mail provider's default SMTP server, Windows Mobile 6.1 automatically loads the value for the alternate SMTP server—which in this case is blank. The e-mail account configuration details are then corrupted, which results in loss of the ability to send e-mail with that particular e-mail account.