If a contact uses a non-Autocrypt e-mail app, all messages involving this contact (in a group or 1:1 chat) will not be end-to-end encrypted, and thus not show a “padlock” with messages. Note that even if your contacts use Delta Chat on their account, they might also use a non-Autocrypt e-mail app on that account which then may cause intermittently unencrypted messages. Replying unencrypted to unencrypted messages is mandated by Autocrypt to prevent unreadable messages on the side of your contacts and their non-Autocrypt e-mail app.
This kind of “a single normal party can accidentally compromise the entire system’s privacy” failure reason is the main reason why email is a pretty bad substrate for E2EE messaging.
This is a pretty big red flag:
This kind of “a single normal party can accidentally compromise the entire system’s privacy” failure reason is the main reason why email is a pretty bad substrate for E2EE messaging.