We had this feature for scanning fro the IMAP folder before, but it was just removed:
There are practically no public keys sent unencrypted in the latest versions of DC, so you will not be able to collect anything other than email addresses this way.
This will just spam all non-DC users with probe emails asking to send back the key. We also normally don’t reply back automatically to anything. During SecureJoin we reply, but only if a known “invite code” from the scanned QR code is sent.