Question about Exchange Online limits in regards of Outlook connectivity


Hello all,

Just a quick question about Exchange Online limits: is there any limit in regards of how many times a EXO inbox can be configured in Outlook client? I have a customer that is having problems when an EXO inbox is configured in Outlook client more than 20 - 25 times? Does any of the Exchange Experts in the house know if this is a know limit? cc @Vasil Michev @Tony Redmond

5 Replies
There are limits, however Microsoft is not publishing those publicly (and they removed the relevant cmdlets years ago). You can get the limits for on-prem Exchange on TechNet, they can serve as educated guess. For example

Best i can do from mobile device, sorry :)

"When an EXO mailbox is configured in Outlook..."  What precisely do you mean when a mailbox is configured? I am unaware of any situations where a configuration change (like changing a mailbox property) would count against some limit. But do tell us exactly what this person is doing so that we can all ponder the great unknown...

Ey Tony, I mean configure Outlook to use a particular mailbox and this mailbox is configured in several PCs/Laptops. According to my customer, after configuring Outlook in 20-25 PCs/Laptops, they start seing th mailbox is not added to Outlook due to some kind of corruption of OST file
best response confirmed by Juan Carlos González Martín (MVP)

So it's a shared mailbox... and yes, once you start to ramp up the number of concurrent accesses to a shared mailbox, you start to encounter performance and other difficulties. Why would they use a shared mailbox for this purpose instead of an Office 365 Group, which will have its own synchronization (to the GST, not OST), and so avoid the issue?

Yeap, it's a shared MailBox...they are using that shared mailbox just because of delegation (SendAs)...something that can also be done with a Group, so this is a great advice and I'm going to suggest to my customer. Thanks Tony!