Whenever I send an email to a Distribution List (approximately 500 users), not all of them are receiving the message. All their SMTP are correct, but I'm always getting this message below and I would like to know what's causing it.
From: Postmaster@domain.com
Sent: Friday, October 16, 2015 3:45 PM
To: Username
Subject: Undeliverable: Unknown
Your message
Subject: Unknown
was not delivered to:
user@tenant.mail.onmicrosoft.com
user@tenant.mail.onmicrosoft.com
user@tenant.mail.onmicrosoft.com
user@tenant.mail.onmicrosoft.com
because:
Error transferring to tenant-mail-onmicrosoft-com.mail.protection.outlook.COM; SMTP Protocol Returned a Permanent Error 500 5.3.3 Unrecognized command
We have a Hybrid/CAS Exchange Online environment. This scenario is sent from Lotus Notes (distribution list) then to Exchange Online, then checks in with our AD FS (Active Directory) then back to Exchange Online and delivered to the users' Outlook if successful, otherwise, the sender will get the Non Delivery Report (NDR).
Also, is there a limit when sending to a Distribution Group? Ours is about 500 users.
Alopez