I too would really like to find a filing app that includes the ability to file outgoing email. Unfortunately, MsgFiler can only file emails that reside in folders such as INBOX or SENT. it does not offer a way to file an outgoing message.
The barrier to adding such a feature has to do with the architecture of Apple's Mail app.I think EFA has the same limitation. Here is the response I got when I asked Dr. Karsten Shulz, the developer, for more info about EFA:
"EFA has a significant user base already and a number of features were inspired by users. We have every intent of maintaining and extending it.
Apple Mail does not have an official plugin architecture that I’m aware of. When we designed EFA, we therefore decided for EFA to be an independent application that communicates with Apple Mail (or Outlook in the case of EFA Pro) through the official Apple Script Interface. This decision has allowed us to follow every single macOS upgrade without significant issues for over 8 years.
Whilst we would like to offer some of the more tightly integrated functions such as EFA as a plugin and send and file, the software architecture of Apple Mail and Outlook would make this a hack that is not guaranteed to work with future versions of macOS. Over the past few major releases of macOS, Apple has tightened the OS system integrity. Without knowing the details, I can only guess that this is what might affect some of the providers that chose a plugin path for their products.
We’ve successfully tested the current versions of EFA/EFA Lite and EFA Pro on Big Sur. Our roadmap is for a re-compiled and adapted official release of EFA/EFA Lite and EFA Pro on Big Sur in early 2021.
I'd be delighted to work with yourself and other Mailhub users on extending EFA, conscious of the technical constraints within the architecture of Apple Mail and EFA as standalone applications.
You are welcome to share what I wrote above with your fellow Mailhub blog buddies.
Wishing you a peaceful and blessed Christmas.