Overview

Some users may experience an issue where calendar invitation responses generate multiple emails, particularly when responses are sent from an iOS device.

Symptoms of this problem include multiple entries in the error-log stating 'More than one attendee is not a valid number.'

Solution

The Development team has thoroughly investigated the issue and found that this behavior is caused by the Native Apple/iOS mail and calendar app itself. Here are a few threads from the Web where users experience the same behavior:

• https://discussions.apple.com/thread/253453268
• https://discussions.apple.com/thread/254726160

The only workaround for this issue is to use Outlook for iOS. Also, you can try using Spark Mail for iOS and see if it works better. 

Alternatively, if you want to continue using the native iOS mail, you could follow the steps below to mitigate the issue:

1. Reconfigure the user's EAS account on the iPhone. This has resolved the same issue for several customers
2. Alternatively, switch from EAS to Integrating IMAP CalDAV or CardDAV Accounts with Kerio Connect on iOS Devices
3. If the issue persists, follow the steps for Resolving Kerio Connect Email Synchronization Issues on Mobile Device

Summary

By reconfiguring the user's account on the iPhone to use Exchange ActiveSync (EAS), the issue of multiple emails being generated from calendar invitation responses should be resolved. If it isn't, there are other sync protocols that could be used. It's important to note that Kerio Connect 10.0.2 and 10.0.2 Patch 1 or subsequent updates will not include a fix for the issue.

FAQ

1. What is Exchange ActiveSync (EAS)?
   Exchange ActiveSync is a Microsoft protocol designed for the synchronization of email, contacts, calendar, tasks, and notes from a messaging server to a mobile device.
2. Are the newest Kerio Connect patches including a fix for this issue?
   No, our Development team has thoroughly investigated the issue and it caused by the iOS updates. There is no code fix to be done in Kerio Connect for this issue.
3. What if the issue persists after reconfiguring the user's account to use EAS?
   If the issue persists, it is recommended to Apple support for further assistance, since the issue started occurring after one of the iOS updates.