Office365: POP and IMAP clients receive OWA links for calendar invitations

As you may know, Office 365 supports a number of different client protocols, including POP and IMAP and a variety of POP and IMAP clients. By default, POP or IMAP clients will be configured to use Outlook Web App (OWA) for handling calendar invitations. When these clients receive a meeting request, within the body of the invite is a link. Clicking on the link allows the user to open their mailbox via OWA so they can accept or decline the request.

If you are using a POP or IMAP client that is capable of handling ICAL messages, you may want to change your configuration settings so that you can get a better experience.

You can configure – on a per-mailbox basis – how POP and IMAP clients receive calendar appointments.  Get-CASMailbox can be used to view your current settings.

Get-CASMAILBOX –identity <NAME> | fl name,*pop*,*imap*

Name                                    : administrator
ExternalPopSettings                     :
InternalPopSettings                     :
PopEnabled                              : True
PopUseProtocolDefaults                  : True
PopMessagesRetrievalMimeFormat          : BestBodyFormat
PopEnableExactRFC822Size                : False
PopSuppressReadReceipt                  : False
PopForceICalForCalendarRetrievalOption  : False
ExternalImapSettings                    :
InternalImapSettings                    :
ImapEnabled                             : True
ImapUseProtocolDefaults                 : True
ImapMessagesRetrievalMimeFormat         : BestBodyFormat
ImapEnableExactRFC822Size               : False
ImapSuppressReadReceipt                 : False
ImapForceICalForCalendarRetrievalOption : False

In the above output, you can see two attributes that you need to set to True to enable iCAL support. The *ICALForCalendarRetrievalOption specifies that the client should be provided calendar appointments that are in ICAL format.

Also note the *UseProtocolDefaults attributes.  These must be set to False in order for any changes to *ICALForCalendarRetrievalOption to take effect.  You can use Set-CASMailbox to change these settings: 

Set-CASMailbox –identity <NAME> –PopUseProtocolDefaults:$FALSE –ImapUseProtocolDefaults:$FALSE –PopForceICalForCalendarRetrievalOption:$TRUE –ImapForceICalForCalendarRetrievalOption:$TRUE

Name                                    : administrator
ExternalPopSettings                     :
InternalPopSettings                     :
PopEnabled                              : True
PopUseProtocolDefaults                  : False
PopMessagesRetrievalMimeFormat          : BestBodyFormat
PopEnableExactRFC822Size                : False
PopSuppressReadReceipt                  : False
PopForceICalForCalendarRetrievalOption  : True
ExternalImapSettings                    :
InternalImapSettings                    :
ImapEnabled                             : True
ImapUseProtocolDefaults                 : False
ImapMessagesRetrievalMimeFormat         : BestBodyFormat
ImapEnableExactRFC822Size               : False
ImapSuppressReadReceipt                 : False
ImapForceICalForCalendarRetrievalOption : True

The settings described here are per-mailbox settings.  There is no global setting to change the default for the entire tenant. There is also no method to adjust the settings per-client.

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