To be totally honest, email already has a problem with mutability – an email which is just a wrapper around remotely hosted images can already be created, though FastMail offers you the option of turning them off or restricting them to senders in your address book.
Uhhhhhh this is virtually every email in your inbox
The email in your mailbox is your copy of what was said, and nobody else can change it or make it go away.
except for cloud providers.
Of course, we’re a hosted “cloud” service. If we turned bad, we could start silently changing your email.
Full standards-compliant access to email. You can use IMAP or POP3 to download messages. IMAP provides the triple of “foldername / uidvalidity / uid” as a unique key for every message. Likewise we provide CalDAV and CardDAV access to the raw copies of all your calendars and contacts.
So does Big Bad Google. Us dinosaurs who still run desktop and mobile clients simply don't have this problem.
Remote server pulls are a way of life. Dynamic remote server pulls are just another way of doing remote server pulls.