[MlMt] address choice per account

[Randy Bush]

> From:  https://manual.mailmate-app.com/hidden_preferences
> 
> The following can be used to add a BCC (blind carbon copy) header by
> default to all outgoing messages:
> 
> defaults write com.freron.MailMate MmDefaultBccHeader -string me at example.com
> 
> Using the special value "<.sender.>" tells MailMate to use the sending
> address of the related account.

thanks.  but /me does not want bcc: on all outbound.  in fact, on almost
none.

randy