jstedfast/MailKit · error · NotSupportedException

The IMAP server does not support the SPECIAL-USE nor XLIST…

Error message

The IMAP server does not support the SPECIAL-USE nor XLIST extensions.

What it means

GetFolder(SpecialFolder) throws NotSupportedException when neither the SPECIAL-USE nor the legacy XLIST capability is present, since mapping well-known folders requires the server to advertise special-use attributes.

Solutions

1. Check (client.Capabilities & (ImapCapabilities.SpecialUse | ImapCapabilities.XList)) != 0 before calling
2. Fall back to GetFolder(client.PersonalNamespaces[0]) and match folders by name (INBOX, Drafts, Sent...)
3. Enumerate client.GetFolders and inspect folder.Attributes for SpecialUse/Sent/Drafts flags where available

Example fix

// before
var drafts = client.GetFolder(SpecialFolder.Drafts);
// after
ImapFolder drafts;
if ((client.Capabilities & (ImapCapabilities.SpecialUse | ImapCapabilities.XList)) != 0)
    drafts = client.GetFolder(SpecialFolder.Drafts);
else
    drafts = (ImapFolder)client.GetFolder("Drafts");

bool canMap = (client.Capabilities & (ImapCapabilities.SpecialUse | ImapCapabilities.XList)) != 0;

try { folder = client.GetFolder(SpecialFolder.Drafts); }
catch (NotSupportedException) { folder = client.GetFolder("Drafts") as ImapFolder; }

When it happens

Trigger: Calling client.GetFolder(SpecialFolder.All/Drafts/etc.) against a server without SPECIAL-USE or XLIST; older servers pre-RFC 6154.

Common situations: Legacy mail servers or minimal proxies; code using SpecialFolder convenience API against servers that only list plain folders.

Related errors

• The IMAP server does not support the QRESYNC extension.
• The IMAP server does not support the UTF8=ACCEPT extension.
• The IMAP server does not support the ID extension.
• The IMAP server does not support the IDLE extension.
• The IMAP server does not support the NOTIFY extension.

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/5836e9456119ed72. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/Net/Imap/ImapClient.cs:2292

		/// The <see cref="ImapClient"/> has been disposed.
		/// </exception>
		/// <exception cref="ServiceNotConnectedException">
		/// The <see cref="ImapClient"/> is not connected.
		/// </exception>
		/// <exception cref="ServiceNotAuthenticatedException">
		/// The <see cref="ImapClient"/> is not authenticated.
		/// </exception>
		/// <exception cref="System.NotSupportedException">
		/// The IMAP server does not support the SPECIAL-USE nor XLIST extensions.
		/// </exception>
		public override IMailFolder? GetFolder (SpecialFolder folder)
		{
			CheckDisposed ();
			CheckConnected ();
			CheckAuthenticated ();

			if ((Capabilities & (ImapCapabilities.SpecialUse | ImapCapabilities.XList)) == 0)
				throw new NotSupportedException ("The IMAP server does not support the SPECIAL-USE nor XLIST extensions.");

			switch (folder) {
			case SpecialFolder.All:       return engine.All;
			case SpecialFolder.Archive:   return engine.Archive;
			case SpecialFolder.Drafts:    return engine.Drafts;
			case SpecialFolder.Flagged:   return engine.Flagged;
			case SpecialFolder.Important: return engine.Important;
			case SpecialFolder.Junk:      return engine.Junk;
			case SpecialFolder.Sent:      return engine.Sent;
			case SpecialFolder.Trash:     return engine.Trash;
			default: throw new ArgumentOutOfRangeException (nameof (folder));
			}
		}

		/// <summary>
		/// Get the folder for the specified namespace.
		/// </summary>
		/// <remarks>

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