Theoretically you could export the certificate and import it on the Server. However it makes more sense to terminate TLS on OPNsense and forward the connection unencrypted or protected with a self signed / internal CA signed certificate. Internal CAs can last longer and OPNsense can refresh the Let's Encrypt certificate automatically so the client will not see any warnings for TLS issues.
Thank you, that's what I had before. But I need the cert on both the OPNsense boxes (for OWA via HAProxy) and on the Exchange boxes (for SMTP + STARTTLS).It's just a couple of clicks and a Powershell script, but I have to remember to do it - so automation would be nice.(I could of course move OWA to another subdomain and have seperate certificates, but the customers are used to mail.domain.com, so I'd rather not.)