When I send emails to anyone, I always sign them using my OpenPGP public key (unless there is a good reason not to do so). This has the dual purpose of allowing future verification of the email and its attachments, should that ever be important, but also to bring into people’s mind the fact that encryption and cryptographic signing of emails is something that is available and can be done (the majority do not even think of it, probably because they are not meant to think of it).
Allow optional (per message) signing of unencrypted emails to non-Delta Chat users, by adding, for example, a “Sign” checkbox on the message compose window.
Allow, as an application-level option, the choice of signing such email messages with either one’s external OpenPGP key or the key used by Delta Chat.
Would there be any reason (besides time and resources, of course) that Delta Chat might not wish to add the optional ability to sign outgoing messages to a non-Delta Chat user, with either (a) one’s normal OpenPGP key (i.e., not the Delta Chat one, therefore requiring password entry); or (b) the Delta Chat key (not requiring password entry)?
A possible issue (particularly with option b) is that one would also need to either attach the public key to the outgoing message, and/or publish the public key on public keyservers.
Another possible issue is that a brief (one-liner?) notice would need to be added to the outgoing message to inform the recipient that the email is signed.
An email to a non-Delta Chat user is always sent both unencrypted and unsigned.