Outlook Mail encryption

Hello,

I have problems with email de-cryption on client email.

We using o365, which has option to Encrypt email.

Client uses other mail server, for example GMAIL.

I'm sending (From Outlook 365) to client simple email with option to Only Encrypt email,

Client gets email to Outlook 2019, that email is ecrypted and it cannot be previewed, and that’s it 😞

I have rad many forums, what i do wrong?

Maybe i must include client email in White list in O365 Admin console / Exchange panel?