It be nice if when the customer emails back to your invoice, letter etc that it comes back into your account for record trail so to say
I think technically this would be difficult to implement as those who use their own SMTP server, you wouldn’t be able to monitor their account for incoming emails.
There is an option at the moment for clients to log in and query invoices etc, which would be logged in your account
It wouldnt be to hard just require settings up imap this side and some rules to filter only certains email address etc