The host name doesn’t match the name specified in the certificate. How can I sort this out?
That certificate belongs to *.jnb2.host-h.net - whoever that is. So you either switch to an insecure connection (not a good idea) or you generate an exemption (trust anyway) for that certificate.
The problem is most likely just a misconfigured email server using the providers global certificate. When you’re sure that host-h.net is acmeaccounting.co.za’s provider just go on an create an exemption. And maybe you should give acmeaccounting’s admins a wake-up call.