ProtonMail for iOS should also act like a well integrated POP3/SMTP client that can send and receive email using Gmail, Outlook.com, and similar services. ProtonMail should look up the recipient’s PGP keys using reputable registries and provide the option of encryption and signing when sending to those parties. All mail should be stored in ProtonMail, and ProtonMail should send the delete call when retrieving via POP3. When onboarding new POP3/SMTP providers, ProtonMail should retrieve and delete from source all email and properly tag each email with a searchable and sortable tag identifying the source. ProtonMail should automatically list PGP keys for these onboarded accounts so they can be retrieved by other ProtonMail users when they send email to these addresses. Those keys should be deleted from the registry after 14 days (with warning) of bad POP3 connection errors (like bad password or unknown user). Those keys should be immediately deleted from the registry if the account is off boarded.