It is best practice to upgrade to Domino 8.5.1 where these features are available through the mail policy settings document (see the link below to "Improving interoperability for users of calendars in e-mail applications other than Notes"). However, if your organization is still using Domino 8.5, you can improve interoperability, depending upon the types of users in your organization, using the following recommendations.

1. If your organization does not require Outlook/Exchange interoperability, set $CSCompatibilityMode=0 on all clients and servers.
2. If you require Outlook/Exchange interoperability but do not want to require end users to select the "Some invitees use other mail clients" option, do one of the following:
• Set DisallowRelatedIcalendarMime=2 on all clients and servers. This causes all non-repeating meetings to work, but complex actions on repeating meetings might not work.
• Set $CSCompatibilityMode=3 on all clients. This causes everything to work seamlessly but restricts repeating meeting options as described above. This setting displays the "Some invitees use other mail clients" option, but it will be automatically selected and dimmed.
3. If you require Outlook/Exchange interoperability and trust users to use the "Some invitees use other mail clients" option properly, do both of the following:
• Set $CSCompatibilityMode=2 in user's NOTES.INI files.
• Consider setting DisallowRelatedIcalendarMime=2 to ensure that meetings work even when the "Some invitees use other mail clients" option is not selected.

For more information, see:

$CSCompatibilityMode
DisallowRelatedIcalendarMime
Improving interoperability for users of calendars in e-mail applications other than Notes