The following issues have been identified when running the Apple built-in apps for mail and calendar with a Traveler Server:

1) Duplicate Sent folder entries

Description:

As of iOS/iPadOS 13.0, Apple devices add an entry to the Sent folder for any emails sent from the Mail app. When the Sent folder is synced, the server entry is added and the device does not remove the original, resulting in a duplicate. HCL development has opened an Apple bug for this issue (FB7337231).

Workaround:
No workaround is available

2) Accepting a meeting invitation from the iOS device does not send the response to the server

Description:
If ghosting is disabled on the Traveler server, responding to a calendar notice from an iOS 13 device does not send the response. The meeting accept is reflected on the app but not in the user's notes calendar.   iOS 13.0 and 13.1 do not send MeetingResponse requests to the Traveler server unless the event is ghosted to the calendar. HCL Development has opened an Apple bug for this issue (FB7328175).

Workaround:
Ghosting is enabled at the Traveler server by default.  

Check the Traveler server notes.ini parameters for NTS_CALENDAR_GHOSTING_SYNCML and NTS_IOS_CALENDAR_INITIAL_GHOST.  

If found, make sure that they are set to true.

Notes:
Known bugs with the iOS Mail, Calendar, and Contacts app working with the Traveler server are documented here:

https://help.hcltechsw.com/traveler/10.0.1/iOSKnownLimitations.html

This information is updated once a fix has been verified in a particular Apple iOS release.