Centralized Mailflow: NDR Remote Server returned ‘550 5.7.1 Unable to relay’


 Environment: Mailbox hosted on the Exchange Online Hybrid on-prem Exchange 2010/2013 with Microsoft Exchange Online Centralized Mailflow configured for Exchange 2013 Route all emails through on-premises configured for Exchange 2010 Accepted domain configured either Managed or Authoritative on the Exchange … Continue reading