If Outlook is used and the CA is installed, the signatures are attributed accurately to Max and Hugo, and are trusted.
If a Thunderbird or an Evolution client is used without the CA installed, then the signatures are attributed accurately, but they are not trusted.
Once your CA is installed in one of these other clients, the signature is correctly attributed and trusted on the first email read regardless of whether it was Max or Hugo. Subsequent emails appear to be "signed" by whoever signed the first email read.
You: Subject: Antw: Astaro (PGP: Plain, Unable to verify signature)
Hugo: Subject: Astaro Encryption (S/MIME: Signed)
You: Subject: Antw: Astaro (PGP: Plain, Unable to verify signature)
Hugo: Subject: Astaro Encryption (S/MIME: Signed)