Set the preferred outgoing mail account at each contact, also allow to set the preferred mail account for a contact group.

I.e.- 

There are two imap accounts configured "Work" and "Play".

Contact "Jack" should always be emailed from the "Work" account.
If a new email is composed, and Jack is entered into the contact, the email sent to Jack should come from the "Work" account. Conversely, emails set to contact "Jill" should always come from the "Play" account.

This would prevent mistakes of quickly composing an email to someone, but forgetting to change the account from which it is sent. An embarrassing and confusing situation when work mails go out from a personal account.

The feature could also be extended so that if an email was sent to both "Jack" and "Jill", the email sent to "Jack" would come from "Work" and the email sent to "Jill" would come from "Play".

Provide a checkbox or option next to the account dropdown to override this feature and send from the account specified (or another method of overriding)