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We have made a change to the way that Exchange Online accounts for the number of subscriptions against the EWSMaxSubscriptions throttling limit.
In earlier versions of Exchange Online, the throttling limit was calculated against the calling account. A calling account, which can be one or many client applications or a service account targeting many mailboxes, was limited to 20 notification subscriptions. This subscription limit restricted Exchange Impersonation scenarios in which a service account accesses a large number of mailboxes. It limited a service account to at most access to 20 different mailboxes, assuming one subscription per mailbox, or 20 subscriptions on one mailbox. This was not very scalable. For example, if a service account had to access 5000 mailboxes, you had to have 250 service accounts to account for this limitation.
Starting with service mailbox versions 14.16.0135 and 14.15.0057.000, this limit has changed. Now, the charges are counted against the target mailbox rather than the calling account. This way, a service account can create subscriptions against many more than 20 mailboxes. One benefit of this change is that you can now design a service application that targets Exchange Online and Exchange on-premises using a similar code base for subscription management. A single service account can now service up to 5000 subscriptions.
Customers, you asked for this change, and now you have it. Subscribe on!