Mac Mail: How can I check incoming Mails for valid certificat with signing

macOS and Mac Apps

Hello, I habe great problems with decrypting mails that are sent by my employer, who uses a gateway mail certificate. The public key of the gateway certificate is installed on my Mac computer (current verson of Catalina). Moreover, I added a key proberty that contains an adress of a colllege, I´d like to transfer encrypted mails with.
There is no problem to send an encrypted mail from my private address to my college - signing and encryption work fine.
However, the way back does not work: when my college wants to send an encrypted mail to me, the contents is not derypted, it seems to be empty.

Can anybody solve my problem?
There is a second problem with VoiceOver in this context_: How can I "see" or listen to the security properties of a mail? Whenever VO reports "security" there are no details reported.