Regular email IMHO shouldn’t make interference with deltachat. The whole point of DC is privacy.
What do you want to “answer” from regular email client to encrypted message if you can’t see content of this message?
No, if DC would make standard email message including email headers (that can contain To field) and encrypt it (wrap the whole message in encrypted container) then DC on receiving side will see all members of group from that internal field To: since it can decrypt it, but unencrypted email header(that visible in plain text for any1) should be send messages as Bcc:
This way man-in-a-middle won’t see all group members but DC will know group members on successful decryption of received message.
Check this Hide group emails from data mining - #5 by AlexJ thread with my proposal