[MlMt] Change default "from" email address behaviour

[vincent at acheson.uk.com]

For my use case, Mailmate’s default behaviour of deciding the sender 
email account based on the addressee email account is causing repeated 
problems: Mailmate’s choice is not always what I expect, and I don’t 
realise the problem until after sending. Is there a configuration 
options which would force me to choose either before I start composing, 
or force me to choose at the point of sending?

Thanks!

Vincent