XMPP - Message arrives "unsafe"

Hello dear ones,

is anyone using XMPP in em client (8.2) (Windows 10)? I have a question, maybe someone of you has experience with it.

I send a message from Android to Android with “Conversations” from the F-Droid Store, the message arrives at the recipient in green “safe”.

If I send from the eM Client (same sender) to Conversations (same recipient), the message arrives there in red “unsafe”.

If the recipient sends a message back to me, it arrives in Conversations, but not in the eM Client.

XMPP provider is mailbox.org
see Introduction to Jabber XMPP - Private Clients - mailbox.org Knowledge Base

Is it possibly because I and the recipient are working without OpenPGP, the eM client does not support OMEMO?

Thanks for feedback in advance.