E-mail system that gives the possibility to catch attachments into mobile app

Hi, we would like to find somebody to assist to make the following possible: While logging to our mobile app for the first time (registration) the user creates a new e-mail adress with our domain extension [login to view URL] User creates e.g.: User1@[login to view URL] Step no. 2: User provides his standard e-mail address this means the e-mail address he uses every day, e.g.: user1@[login to view URL] and on this mail all incoming messages (from user1@[login to view URL]) will be forwarded to user1@gmail.com. We want to use our domain/e-mail (user1@[login to view URL]) just to grab the attachments, that means import attachements/picurest/docs into our app so that the users have it immediately on his mobile within our app. We don't want to create a new mailing system to send or write messages, we just want to catch the attachements into our app and forward on his mail so he finally receives the mail on the user's main e-mail address and doesn't have to use several mail addresses. Who could help? Now we are using g-suite for e-mails maybe the configuration above is possible with some adjustments/configuration within google g-suite?