office365 – MS Graph API: Event list api for external users in office 365 or Azure Active Directory

I have implemented MS Graph API in my SaaS based application through which user can login from their account and see their calendar events and add them too. My organization user and my personal account user are able to use it. But external domain user weren’t able to login or manage calendar. So I have added them as an external user in my organization now they are able to login but list_event API which gives all the calendar events giving me this error.

 {\"error\":{\"code\":\"MailboxNotEnabledForRESTAPI\",\"message\":\"The mailbox is either inactive, soft-deleted, or is hosted on-premise.\"}}

I have checked all the permissions my permission for the app are these.
“User.ReadBasic.All” , “Mail.ReadWrite” , “Calendars.Read”, “Calendars.ReadWrite” , “Mail.Send”, “Mail.Read”

One thing which I have checked that my external users are not having O365 license in my admin center and I can the license as well from my admin center but after that this issue is also persist. But when I remove license from one of my organization user than this issue occur for that user as well. So in my opinion O365 license should be used for external users but license should already been added by their organization because they can access their mails.