I would try to solve your use case this way:
Using the option “Sender based on Reply-To header”, a received mail can be forwarded to zammad with the customer mail as Reply-To.
This way zammad would create the new customer based on the correct email.
Yes, probably you’ll need to set the Reply-To, but you’ll only need to send one mail.
To inform the customer, let zammad do this via trigger.
This way the customer can be informed automatically, including the corresponding ticket number and thus making follow up detection possible.
Alternate possibility:
After forwarding the mail to zammad, let the agents send their replies from within zammad, not from their personal email client.
This way the follow up detection handles everything itself.
Zammad won’t create a new user.
I hope this helps you a bit?