It is known that a single Microsoft Exchange account is not sending more than 10k emails per day.

It is also know that once activated – Microsoft 365 groups ownerless policy will be sending notifications for all groups in scope to specified number of group members within 24 hours.

The question is: what if there are more than 10,000 notifications to send (e.g. 4,000 ownerless groups and the policy is configured to send notification to 3 members per group – that gives us 12,000 notifications to send)? Would the policy send 10k notifications and the rest 2k notifications the next day?

I’m conducting an experiment. I created 10k groups in my lab tenant with one owner and 3 random members. Then I configured a policy that is sending notification to a 3 most active members (in this case – random members). And then I made all these groups ownerless by deleting the single owner Id from Azure AD (Microsoft Entra).

Here is what I got from users perspective:

user	user groups number	got messages day 1	got messages day 2	got messages total
1 Roger	5012			1374
2 Dick	5039			1349
3 Bob	5108			3412
4 Bapu	4908			1376
5 Stas	4996			1437
6 David	4959			1377
total				10325

Here is what audit log says:

Events “OwnerlessGroupNotified” day 1: 4949
Events “OwnerlessGroupNotified” day 2: 95
Events “OwnerlessGroupNotified” total: 5044
Each event details says 3 members were notified.

It seems like groups are selected by policy in random order.

Massive E-mails sending was started 43 minutes after midnight UTC

“OwnerlessGroupNotified” were logged at the rate of
1925 events during 1-st hour,
2029 events during 2-nd hour,
785 events during 3-rd hour,
176 events during 4-th hour,
26 events during 5-24 th hour,
95 events during next 25-48 hours
so max rate was one event every 3 seconds in the beginning (or 1 e-mail per second) …

TBC…