I'm hoping to add a auto-signature to my outgoing emails.

@Synthyx Hi there! Tuta is fully encrypted - not just the emails, but also the calendar and your entire address book (which you can sync to your phone and use it as an encrypted contacts' app). To support external clients, we would need to support IMAP. But this would mean we would have to send the data unencrypted to a third-party client (e.g. Microsoft's Outlook or Apple Mail or Thunderbird), which defies our security promise of quantum-safe end-to-end encryption.