jstedfast/MailKit · error · NotSupportedException

The ImapFolder does not support mod-sequences.

Error message

The ImapFolder does not support mod-sequences.

What it means

CheckCanFetch(IList<UniqueId>, IFetchRequest) — used by Fetch/FetchAsync by UID — throws this NotSupportedException when the fetch request sets ChangedSince but the folder's supportsModSeq flag is false. Fetching only messages changed since a mod-sequence requires the CONDSTORE extension (RFC 7162); MailKit validates this client-side instead of letting the server return BAD.

Solutions

1. Check folder.SupportsModSeq before setting request.ChangedSince; if false, fall back to a full Fetch and diff client-side
2. Verify the ImapClient advertises CONDSTORE/QRESYNC (client.Capabilities) and that the server has the extension enabled
3. Clear the ChangedSince value on the IFetchRequest when unsupported

Example fix

// before
var request = new FetchRequest { Items = MessageSummaryItems.All, ChangedSince = lastModSeq };
var summaries = folder.Fetch(uids, request);
// after
if (folder.SupportsModSeq) {
    request.ChangedSince = lastModSeq;
} else {
    request.ChangedSince = null; // full fetch + client-side diff
}
var summaries = folder.Fetch(uids, request);

if (request.ChangedSince.HasValue && !folder.SupportsModSeq)
    request.ChangedSince = null; // full fetch + client-side diff
var summaries = folder.Fetch(uids, request);

try {
    summaries = folder.Fetch(uids, request);
} catch (NotSupportedException) {
    request.ChangedSince = null;
    summaries = folder.Fetch(uids, request); // fallback full fetch
}

When it happens

Trigger: Calling Fetch(IList<UniqueId> uids, IFetchRequest request) or FetchAsync where request.ChangedSince.HasValue and the IMAP server does not support CONDSTORE/QRESYNC (supportsModSeq == false).

Common situations: Implementing incremental sync (pull only changed messages since last known modseq) against Gmail, which lacks CONDSTORE; setting ChangedSince copied from sample sync code without checking capabilities; reconnecting to a different/misconfigured server that lost CONDSTORE.

Related errors

• The IMAP server does not support the PARTIAL extension.
• The set of unique identifiers is too large to fetch with a…
• The PARTIAL extension only supports UID-based FETCH…
• The ImapFolder does not support mod-sequences.
• The IMAP server does not support the METADATA extension.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/ab17f6ea8690c924. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapFolderFetch.cs:1096

				MessageExpunged -= sctx.OnMessageExpunged;
			}
		}

		internal static bool IsEmptyFetchRequest (IFetchRequest request)
		{
			return request.Items == MessageSummaryItems.None && (request.Headers == null || (request.Headers.Count == 0 && !request.Headers.Exclude));
		}

		bool CheckCanFetch (IList<UniqueId> uids, IFetchRequest request)
		{
			if (uids == null)
				throw new ArgumentNullException (nameof (uids));

			if (request == null)
				throw new ArgumentNullException (nameof (request));

			if (request.ChangedSince.HasValue && !supportsModSeq)
				throw new NotSupportedException ("The ImapFolder does not support mod-sequences.");

			if (request.PartialRange.HasValue && (Engine.Capabilities & ImapCapabilities.Partial) == 0)
				throw new NotSupportedException ("The IMAP server does not support the PARTIAL extension.");

			CheckState (true, false);

			return uids.Count > 0 && !IsEmptyFetchRequest (request);
		}

		string CreateFetchCommand (IList<UniqueId> uids, IFetchRequest request, out bool previewText)
		{
			var query = FormatSummaryItems (Engine, request, out previewText);
			var command = new StringBuilder ("UID FETCH %s ");

			command.Append (query);

			if (request.ChangedSince.HasValue || request.PartialRange.HasValue) {
				command.Append (" (");

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