Exchange Hybrid: User in O365 cannot open on-premise secondary mailbox (only from external networks)

Senior Member



I'm currently facing a problem for which I cannot find a solution. (Ok it's obvious because I won't be there if I had a solution ;) )


We have an Hybrid installation of O365


The problem is between

- User in O365

- Secondary mailbox On-Premise


Context: In Outlook 2016

- I have my own mailbox as primary (on O365)

- I have two secondary mailboxes On-Premise (Exchange 2016)

- I have two secondary mailboxes (O365)



- Connected to the internal network of the company, each mailboxes are accessible without any issues



- Connected outside my organization, only my own mailbox and secondary mailbox on O365 are available

- The 2 secondary mailboxes On-premise are not accessible and I have the standard error message "Cannot expand the folder. The set of folder cannot be opened. etc."


So I guess there is a problem on our firewall who is blocking something between O365 and On-Premise but I did not see anything on the monitoring of this firewall


Does anyone have an idea of what could be the problem on my issue ?


Thanks in advance



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