554 5.7.1 Rejecting email with relay denied
If either an external party or an email system reports "554 5.7.1 Rejecting email with "From" header as where sender IP/host is not authorised to relay denied" when sending an email into your organisation via Mailsphere then the issue will be resolved through the domain configuration within Mailsphere.
The incoming message will be sent from a domain that is managed by Mailsphere. As such all IP addresses that send on behalf of this domain must be included in the domain configuration. This can be found here:
Using the Incoming Addresses ensure that all IP addresses of your email system end points as well as any 3rd party solutions that send email on your behalf have been included. It is possible to include IP4/IP6 as well as address ranges and FQDN.
The 554 5.7.1 delivery failure message will contain the IP address and the FQDN of the MTA that attempted to deliver the email that was rejected.