I have a particular contact who uses Delta Chat, too, with whom I have chatted for a while without problems.
At some point, I still received their messages while they stopped to receive mine. This persists for several Delta Chat versions on both sides. My messages appear as always in my outgoing folder, so everything looks fine on my side.
I know that this person has been switching devices recently and I wondered if they maybe haven’t synchronised their keys. Even if I have the wrong key, this probably isn’t the cause of the problem as the messages don’t seem to arrive at all, instead of just being undecipherable. But this made me wonder:
What does Delta Chat do if an encrypted chat was started with one key of the other side and then it continues with a new key?