All forwarded chat messages are stored in the database with the email address you've forwarded them with. The database links your email address to the forwarded chat messages. This coupling ensures that only you can read your own chat messages and not the chat messages from someone else and vice versa.
I.e. When you forward a chat messages from WhatsApp using the email address "firstname.lastname@example.org", then you must also register with "email@example.com" for a new SaveMMS account.