Is there a documented maximum number of users you can have connected to an Office 365 user mailbox?

We have a user mailbox on Office 365 that we share out to a department at our company with around 20 users. We've had some problems with the mailbox regularly updating for some users.

Utilizing a Shared Mailbox is not an option due to the size and required functionality of the mailbox (sending out emails via TLS through O365).

We're being told by our Vendor of Record that provides support (as a middleman) for Office 365 that you cannot have that many users connected to a user mailbox with Office 365 but I cannot locate any documentation to support such a claim.

Thanks in advance for any assistance.

I received the following feedback from Microsoft with regards to throttling in place for Office 365:

The throttling limit for attribute “RCAMaxConcurrency”, which defines the number of concurrent connections allowed from Outlook, is set to 40 which translates to roughly to roughly 14 max conncections from Outlook 2007/2010/2013 clients to the mailbox.

However, Microsoft also recommended a workaround for this would be to use a Shared mailbox which would get around this problem as Shared mailboxes are not handled the same as User mailboxes on Office 365.

Hope this helps somebody else in the future!

It's not a hard limit of endpoints or users, it's about throttling policy settings. Each endpoint, whether Outlook (MAPI), or mobile (EWS typically), will use a number of connections. That number may be 1, or it may be higher. Each connection will do certain things like connect to AD, or search, or send mail, etc. When you hit a throttling limit for a type of behavior over a type of connection, future connections will be throttled.

(User vs Shared mailboxes will make no difference to O365. It's still a mailbox.)