When transports defined in the settings have become corrupted, you can still select them from the Transport pull-down and the composer will behave as if the selected transport is being used, but will actually use the default transport instead without warning the user.

The silence of the failure causes a minor security leak for people who use transports to keep different life-roles separate (eg, have a transport for work mail separate from one for personal use and don't want to send messages from one via the other). I had sent several messages via transports I did not wish them to go via before I noticed, potentially compromising my privacy :-(

Reproducible: Always

Steps to Reproduce:
1. Have a number of transports set up. Corrupt them (not sure how mine became corrupted - likely a bad upgrade conversion).
2. Select a non-default transport.
3. Send message.
4. Observe message in 'sent' box was sent by default transport
Actual Results:  
Mail is sent via default transport instead of selected transport without any sign to user that this has happened.

Expected Results:  
Composer should warn user that it cannot use the selected transport (and suggest re-setting up the transport in preferences).