Updated Requirements for SMTP Relay through Exchange Online

Hi,

Something we've noticed while working on preparing our tenant for this change - we've noticed an issue with our third party SMTP relays that are our last hop to the internet when sending, and first hop for receiving (MX records point to these releays and not O365).  If these external relays are added to a connector in Office 365 ('partner' or 'your org' type), we fairly quickly see email destined for other tenants on the same 'smart host' infrastructure (<GUID>.mail.protection.outlook.com IPs resolve to the same or as on the same subnet as our tenants <GUID>.mail.protection.outlook.com 'smart host' resolve to)  - Our receive connectors for these relays appear to 'kick in' when we send email to the other tenant.  This results in the SMTP relay eventually rejecting the email because it's looping.  To our relay, we hand it off to 'otherdomain.mail.protection.outlook.com' but it shows up in our tenant, EOP rules send it to the external relay (again) to deliver to external recipients, which routes  it to 'otherdomain.mail.protection.outlook.com' and it shows up in our tenant again...repeat until the relay stops the loop.  Opened a case with support and it went no where.  Only solution for us was to not use a connector for these relays to Office 365 and hope for the best.  Anyone else seen this issue?  We're expecting at some point not having a connector to cover these relays connections is going to cause us an issue.