Apple Calendar Silently Fails synchronisation on more than 2000 events (default WebDAV Value)

[mtthidoteu]

Issue Description

When a CalDAV calendar contains more than 2000 events, Apple Calendar (iOS/macOS) fails to sync all events. This is because the sync-collection REPORT response is truncated at the dav.response.max-results limit (default: 2000) with a 507 Insufficient Storage status and <D:number-of-matches-within-limits/> error. Apple Calendar does not follow up with the
returned sync-token to fetch the remaining events — it silently drops them.

While this is partly an Apple Calendar client bug (not paginating), the low default limit and silent truncation make this a significant footgun for any user with a large calendar history and an IOS device. This is a major issue.

Expected Behavior

All calendar events should be synced to CalDAV clients regardless of collection size. Either:

1. The default dav.response.max-results should be higher (e.g. 10000), or
2. The setting should be more prominently documented with a warning about client compatibility, or
3. Stalwart should log a warning when a sync-collection response is truncated

Actual Behavior

With ~2950 events in a single calendar, the sync-collection REPORT returns only 2001 entries and a 507 status:

<D:response>
<D:href>/dav/cal/user%40example.com/default/</D:href>
<D:status>HTTP/1.1 507 Insufficient Storage</D:status>
<D:error><D:number-of-matches-within-limits/></D:error>
<D:responsedescription>The number of matches exceeds the limit of 2000</D:responsedescription>
</D:response>

Apple Calendar accepts the first 2000 events and ignores the rest. The user sees a partial calendar with no error or indication that events are missing. Apple Calendar silently fails.

Reproduction Steps

1. Import >2000 events into a single CalDAV calendar on Stalwart (default dav.response.max-results = 2000)
2. Add the CalDAV account to Apple Calendar on iOS or macOS
3. Observe that only ~2000 events are visible — the remainder are silently missing
4. Increasing dav.response.max-results to a higher value resolves the issue AFTER deleting and adding the account

Relevant Log Output

No error logged server-side. The truncation is silent from the admin's perspective.

Stalwart Version

v0.15.x

Installation Method

Binary (Linux)

Database Backend

RocksDB

Blob Storage

RocksDB

Search Engine

Internal

Directory Backend

Internal

Additional Context

Apple Calendar

I acknowledge that:

• I have reviewed the documentation and FAQ and confirm that my issue is NOT addressed there.
• I have searched the Stalwart repository (both open and closed Discussions and Issues) and confirm this is not a duplicate.
• I have set the logging level to trace and included relevant log output if applicable.
• I agree to follow the project's Code of Conduct.

[skyflyer]

I had the same situation and spent a lot of time troubleshooting and figuring out what ti was. Actually, I had several issues combined:

• rate limit issue
• address book (card entry) size limit (I have one bigger than 2MB)
• item limit (I have more than 8000 calendar entries)

I used vdirsyncer with debug logging and still couldn't easily figure out why the calendars (Apple, Thunderbird and BusyCal) were not displaying some entries even though they were present on the server (verified and checked manually with curl commands to query dav directly).