550 5.1.90 NDR

akpvenus It was long time ago and I can't recall the root cause / solution anymore. But in fact, I didn't struggle with this issue anymore. I have found some notes:

Recipient Limits 

There are two recipient limits that count per-user to all outbound and internal messages 

1. Recipient rate limit – all recipients over last 24 hrs – 10 000 (not configurable) 

1. Recipient limit – number of recipients in a single message. – max. 1000 (configurable by admin) 

Clarification for Distribution Groups / Lists:  

For the purposes of the recipient rate limit and the recipient limit, a distribution group that is stored in the organization's shared address book counts as one recipient.  

In a personal distribution list, each recipient is counted separately. 

NDR 

Remote Server returned '550 5.1.90 Your message can't be sent because you've reached your daily limit for message recipients." 

DOCS: 

All limits: https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#sending-limits 

All NDR codes: https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/non-delivery-reports-in-exchange-online