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Weird behavior of Exchange Online for none-existing email address
I am see a weird behavior of Exchange Online for none-existing email address, wondering if anyone can explain it. I am setting up a Fortimail Cloud service ( third party email protection service ...
- Jack_Chen1780 - This behavior is expected. The DBEB functionality can reject on either RCPT TO or on "End Of Data". The latter happens when the sending IP is shared between multiple tenants, and in order for EOP to correctly attribute to the recipient tenant, we need to traverse the headers which are available only after EoD.
MS is likely handling messages coming from Fortimail differently. The second scenario is what you'd expect to happen (or a 550 5.7.64 TenantAttribution; Relay Access Denied depending on whether the domain is known to EOP and how its configured). In any case, best open a support request.
Indeed, It looks like a issue with Exchange Online Canada service, it might have some special rules for Fortimail Cloud.
I searched Microsoft Partners and found two partners using Exchange Online and does send 550 for none-existing users :
Sherweb.com : MX sherweb-com.mail.protection.outlook.com, Canada.
carahsoft.com: MX carahsoft.mail.protection.outlook.com. US.
Test result from Fortimail Cloud :
Opened a ticket with Fortinet, they are investigating, hopefully they can work it out with Microsoft.