Hi, We are also experiencing the same issues.
With one difference that we have managed to get it working on two other servers at other locations.
So we have three locations and at two of these locations the servers do what they are supposed to do, both sending and receiving invitations to appointments both locally and from outside the office on iPads, iPhones and laptops.
The new server I have just run up simply won't do remote/external invitations, however internal invitations work. I have copied and pasted below the error log from the server showing the error that is occurring. I am hoping somebody can tell me what I am doing incorrectly. Very weird as I think I copied the same processes from the last two installs.
2016-06-06T14:42:50+1000 [caldav-0] [txdav.caldav.datastore.scheduling.imip.smtpsender#error] Failed to send IMIP message <20160606044211.5037.2021263278.0@samaras-mac-mini.local> from rick@iapps.net.au to mailto:manager@imagineit.com.au (Reason: Failure with multiple causes.)
I have done a entire reinstall of the server and reinstalled the latest system and server software. so System 10.11.5 and Server 5.1.5 the other severs I am running are also on the same software versions.
I have copied a little more of the error log below just incase the error is deeper than I thought.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseSchemaStep#warn] Beginning database schema check.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseSchemaStep#warn] Required database key VERSION: 60.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseSchemaStep#warn] Actual database key VERSION: 60.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseSchemaStep#warn] Schema version check complete: no upgrade needed.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseSchemaStep#warn] Database schema check complete.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseAddressBookDataStep# warn] Beginning database addressbook data check.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseAddressBookDataStep# warn] Required database key ADDRESSBOOK-DATAVERSION: 2.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseAddressBookDataStep# warn] Actual database key ADDRESSBOOK-DATAVERSION: 2.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseAddressBookDataStep# warn] Addressbook data version check complete: no upgrade needed.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseAddressBookDataStep# warn] Database addressbook data check complete.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseCalendarDataStep#war n] Beginning database calendar data check.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseCalendarDataStep#war n] Required database key CALENDAR-DATAVERSION: 6.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseCalendarDataStep#war n] Actual database key CALENDAR-DATAVERSION: 6.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseCalendarDataStep#war n] Calendar data version check complete: no upgrade needed.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseCalendarDataStep#war n] Database calendar data check complete.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseNotificationDataStep #warn] Beginning database notification data check.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseNotificationDataStep #warn] Required database key NOTIFICATION-DATAVERSION: 1.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseNotificationDataStep #warn] Actual database key NOTIFICATION-DATAVERSION: 1.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseNotificationDataStep #warn] Notification data version check complete: no upgrade needed.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseNotificationDataStep #warn] Database notification data check complete.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseOtherStep#warn] Beginning database other upgrades check.
2016-06-06T15:50:14+1000 [txdav.common.datastore.upgrade.sql.upgrade.UpgradeDatabaseOtherStep#warn] Database other upgrades check complete.
2016-06-06T15:50:14+1000 [twistedcaldav.authkerb.NegotiateCredentialFactory#error] getServerPrincipalDetails: ('Principal not found in keytab', -1)
2016-06-06T15:50:14+1000 [twext.enterprise.jobs.queue#error] workCheck: jobqueue is overloaded
2016-06-06T15:50:19+1000 [twext.enterprise.jobs.queue#error] workCheck: jobqueue is no longer overloaded
2016-06-06T16:03:35+1000 [caldav-1] Reading configuration from file: /Applications/Server.app/Contents/ServerRoot/private/etc/caldavd/caldavd-apple. plist
2016-06-06T16:03:35+1000 [caldav-1] Adding configuration from file: /Library/Server/Preferences/Calendar.plist
2016-06-06T16:03:35+1000 [caldav-1] Adding configuration from file: /Library/Server/Calendar and Contacts/Config/caldavd-system.plist
2016-06-06T16:03:35+1000 [caldav-1] Missing configuration file: /Library/Server/Calendar and Contacts/Config/caldavd-user.plist
2016-06-06T16:03:35+1000 [caldav-1] [txdav.caldav.datastore.scheduling.caldav.scheduler#error] Unknown calendar user address: mailto:rick@tdove.net
2016-06-06T16:03:35+1000 [caldav-1] [txdav.caldav.datastore.scheduling.scheduler.ScheduleResponseQueue#error] Error during PUT for mailto:rick@tdove.net: Unknown recipient
2016-06-06T16:03:35+1000 [caldav-1] [txdav.caldav.datastore.scheduling.caldav.scheduler#error] Unknown calendar user address: mailto:manager@imagineit.com.au
2016-06-06T16:03:35+1000 [caldav-1] [txdav.caldav.datastore.scheduling.scheduler.ScheduleResponseQueue#error] Error during PUT for mailto:manager@imagineit.com.au: Unknown recipient
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Could really do with some help, I have been on this for days and i can't seem to get it working.