How to configure O365 Calendar for On-Prem custom Mail solutions

[pavanpavuluri]

We have few users that use custom mail solution. Is there a way to configure Teams calendar for these users without moving them to exchange?

[#2 · Accepted solution]

Unfortunately no. Teams doesn't actually have a calendar, it leverages Exchange / Exchange Online mailboxes. The closest you could get would be to build a solution to sync entries between an Exchange Online mailbox and your custom solution using Graph API.

https://docs.microsoft.com/en-us/graph/api/resources/calendar?view=graph-rest-1.0