For those Exchange Online Administrators who have and/or send/organize large Calendar Invitations (500 or more recipients), then this posting is for you!
..special thanks to Folke for pulling this information into a great format!
Office 365 : Exchange Online
|Sending Calendar Meetings to more than 500 recipients with Exchange Online, it is HIGHLY RECOMMENDED that the usage of an Exchange Online Distribution List (DL) is used, as groups are counted as 1 recipient.In Exchange Online, the recipient limit per message is restricted to a maximum of 500, which is why EXO DLs should be used. When and if the recipients are > 500, the Meeting Organizer is unable to properly send updates, cancellations, etc, due to the 500 limit.||Turn off the large calendar meeting option for Request Response which will allow users to respond, however they will not have the ability to send “comments”.The disablement of this Request Response is needed in Exchange Online meetings so that respondents are not pulled out of the DL and placed onto the To line, which starts the counter towards the EXO 500 recipient limit: Office 365: Recipients and Sender limits
Note -Due to the fact that certain mobile devices, such as an HTC Android, does not adhere to this Request Response setting. As a result these devices and the respondent will be added to the To line. Meeting Organizers should be careful in that if you have ~500 HTC Androids that respond to this meeting invite, the Meeting Item will be come unmanageable.
|Should use this process going forward for large EXO Large Calendar Invitations.|
Solution: Use EXO DLs and Turn Off “Request Response”
When sending these large calendar invitations in Exchange Online (i.e. All Hands Meeting), you should:
- Create and use EXO Distribution Lists (DLs) as recipients in the large calendar item
- Disable the Request Response flag in the Outlook meeting, so users are not pulled out of the DL and placed onto the To line, starting the 500 Max Recipients/Message EXO limit.
Exchange Online has a restriction of a max of 500 recipients per message to prevent Denial of Service Attacks against the service.
When you leave the standard setting of Request Response=on, attendees can respond not only with Accept, Tentative and Decline, but also with comments.
If an attendee responds with comments, that attendees email address is placed on the TO line, thus outside of the DL and counts as one recipient.
If this happens more than 500 times, the max recipients exceed the 500 limit (1DL+ 500 recipients on TO line=501 recipients = limit exceeded). Now the Meeting Organizer cannot properly manage the Calendar Meeting Invite, because they cannot send updates because there are more than 500 people on the To line.