How to exchange encrypted messages between DC and another email client

I want to send encrypted messages from DC to a person that does not use DC. Before writing a message to them, I tested it with one of my email addresses. The following tests are between my DC email address (mydc@someserver.tld) and my normal email address using non-DC clients (mymail@anotherserver.tld). I am using a separate email address for DC.

First, I tried an email client that does not support Autocrypt. I extracted the public key of my DC client manually from the email header. The email client was then able to send an encrypted message to DC. DC however did not show the lock icon next to the message and was unable to respond with an encrypted message, probably because the email it received did not include the public key in the header (since the sending email client did not support autocrypt).

Then I switched to an email client that supports autocrypt. I was able to send an encrypted message to DC. The DC Android app still did not show the lock icon. In the contact details menu it showed the fingerprint of the key though, so I guess DC was able to extract the public key from the email header. It refused to send encrypted emails though.

So now I’m running out of ideas what else I could try to convince DC on Android to send an encrypted email to my other email address. What do I need to do? What else can I try?

DC on Android is version 1.14.1 from F-Droid. The desktop client (Linux) version 1.13.1 doesn’t do it either, showing the same symptoms.
The email client without autocrypt is KMail version 5.7.3 on Kubuntu 18.04.5.
The email client with autocrypt is K9-Mail on Android version 5.600.

Sounds like the message was encrypted but not signed.

Also, maybe there is a setting like “Prefer end-to-end encryption” in the email client that supports autocrypt?

Sadly there is no such option in the client I used (K9-Mail on Android). It can only toggle autocrypt on or off, not set a specific mode of operation.

Does K9 show you whether the outgoing email (K9 to DeltaChat) is encrypted and signed?