jstedfast/MailKit · error · InvalidOperationException

The MessageNew event cannot have any parameters for mailbox…

Error message

The MessageNew event cannot have any parameters for mailbox filters other than SELECTED and SELECTED-DELAYED.

What it means

The MessageNew event can carry a fetch request (summary items) only when the event group's mailbox filter is SELECTED or SELECTED-DELAYED; for other filters (Personal, Subscribed, mailboxes list) RFC 5465 forbids parameters on MessageNew. Format() throws InvalidOperationException if a request is attached and isSelectedFilter is false.

Solutions

Remove the parameters so MessageNew is parameterless for non-SELECTED filters (new ImapEvent.MessageNew() or ImapEvent.MessageNew as appropriate)
Use MailboxFilter.Selected/SelectedDelayed if you genuinely need MessageNew with summary items and only care about the currently selected mailbox
Fetch full summaries yourself in the notification handler after receiving the event

Example fix

// before
var group = new ImapEventGroup(ImapMailboxFilter.Personal) {
    Events = { new ImapEvent.MessageNew(MessageSummaryItems.UniqueId | MessageSummaryItems.Envelope) }
};

// after
var group = new ImapEventGroup(ImapMailboxFilter.Personal) {
    Events = { ImapEvent.MessageNew, ImapEvent.MessageExpunge }
};

Defensive patterns

Strategy: validation

Validate before calling

bool MessageNewParamsAllowed(ImapEventGroup g) =>
    g.MailboxFilter is ImapMailboxFilter.Selected or ImapMailboxFilter.SelectedDelayed
    || g.Events.All(e => e is not ImapEvent.MessageNew mn || ImapFolder.IsEmptyFetchRequest(mn.Request));

Type guard

null

Try / catch

try {
    client.Notify(groups);
} catch (InvalidOperationException ex) when (ex.Message.Contains("MessageNew event cannot have any parameters")) {
    // rebuild with parameterless MessageNew for non-selected filters
}

Prevention

Only attach summary-item requests to MessageNew in SELECTED/SELECTED-DELAYED groups
Check the filter type before configuring MessageNew parameters
Handle new-message data needs by fetching in the event handler instead

When it happens

Trigger: Creating an ImapEventGroup with MailboxFilter = Personal (or Subscribed/Mailboxes) and setting the MessageNew event's request items (e.g. via new ImapEvent.MessageNew(summaryItems)), then issuing NOTIFY.

Common situations: Copy-pasting a SELECTED-group MessageNew configuration into a Personal group; wanting preview/summary data for new messages across all personal folders, which NOTIFY does not allow.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/bb34c8cf0135c704. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapEventGroup.cs:725

			/// </summary>
			/// <remarks>
			/// Formats the IMAP NOTIFY command for this particular IMAP mailbox filter.
			/// </remarks>
			/// <param name="engine">The IMAP engine.</param>
			/// <param name="command">The IMAP command builder.</param>
			/// <param name="args">The IMAP command argument builder.</param>
			/// <param name="isSelectedFilter"><see langword="true" /> if the event is being registered for a
			/// <see cref="ImapMailboxFilter.Selected"/> or <see cref="ImapMailboxFilter.SelectedDelayed"/>
			/// mailbox filter.</param>
			internal override void Format (ImapEngine engine, StringBuilder command, IList<object> args, bool isSelectedFilter)
			{
				command.Append (Name);

				if (ImapFolder.IsEmptyFetchRequest (request))
					return;

				if (!isSelectedFilter)
					throw new InvalidOperationException ("The MessageNew event cannot have any parameters for mailbox filters other than SELECTED and SELECTED-DELAYED.");

				command.Append (' ');
				command.Append (ImapFolder.FormatSummaryItems (engine, request, out _, isNotify: true));
			}
		}
	}
}

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