Oh mono, you make this too easy. I was wondering if I could write an IMAP client from scratch in MonoTouch. Turns out yes, and turns out that it's easy to do!

ImapClient.cs

// Authenticating [email protected]
// S: * OK Gimap ready for requests from 64.225.210.112 oi7if5386254oac.123
// C: A0001 LOGIN [email protected] ****************
// S: * CAPABILITY IMAP4rev1 UNSELECT IDLE NAMESPACE QUOTA ID XLIST CHILDREN X-GM-EXT-1 UIDPLUS COMPRESS=DEFLATE
// S: A0001 OK [email protected] Frank Krueger authenticated (Success)

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.IO;
using System.Net.Security;
using System.Net.Sockets;
using System.Security.Cryptography.X509Certificates;
using System.Text;

namespace Inbox
{
  public class ImapClient
	{
		public string Host { get; set; }
		public int Port { get; set; }

		public string Email { get; set; }
		public string Password { get; set; }

		int nextReqId = 1;

		SslStream stream;
		byte[] readBuffer;
		MemoryStream responseBuffer;

		public ImapClient ()
		{
			readBuffer = new byte[4 * 1024];
			responseBuffer = new MemoryStream ();
		}

		public void Connect ()
		{
			TcpClient client = new TcpClient (Host, Port);
			stream = new SslStream (
				client.GetStream (),
				false,
				ValidateServerCertificate,
				null);

			Debug.WriteLine ("SSL Authenticating with " + Host);

			stream.AuthenticateAsClient (Host);

			BeginRead ();

			Authenticate ();
		}

		void BeginRead ()
		{
			stream.BeginRead (readBuffer, 0, readBuffer.Length, OnRead, null);
		}

		void OnRead (IAsyncResult ar)
		{
			var numRead = stream.EndRead (ar);

			if (numRead < 0) {
				Debug.WriteLine ("Read failed with n = " + numRead + ". Reconnecting.");
				Connect ();
				return;
			}

			//
			// Look for \r\n (13,10)
			//
			var i = 0;
			while (i < numRead - 1) {
				if (readBuffer[i] == 13 && readBuffer[i+1] == 10) {
					//
					// Got it, paste it onto responseBuffer
					//
					responseBuffer.Write (readBuffer, 0, i + 2);
					responseBuffer.Flush ();

					HandleResponse ();

					//
					// Begin looking for the next response
					//
					numRead -= (i + 2);
					if (numRead > 0) {
						Array.Copy (readBuffer, i + 2, readBuffer, 0, numRead);
					}
					i = 0;
				}
				else {
					i++;
				}
			}

			//
			// And continue...
			//
			BeginRead ();
		}

		void HandleResponse ()
		{
			var data = responseBuffer.ToArray ();
			var str = Encoding.UTF8.GetString (data);

			Debug.WriteLine ("S: " + str);

			responseBuffer.Position = 0;
			responseBuffer.SetLength (0);
		}

		void Authenticate ()
		{
			Debug.WriteLine ("Authenticating " + Email);

			Command ("LOGIN " + Email + " " + Password);
		}

		void Command (string line)
		{
			var realLine = string.Format ("A{0:0000} {1}\r\n", nextReqId, line);
			nextReqId++;

			Debug.WriteLine ("C: " + realLine);

			var lineBytes = Encoding.UTF8.GetBytes (realLine);

			stream.Write (lineBytes);
			stream.Flush ();
		}

		bool ValidateServerCertificate (object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
		{
			return true;
		}
	}
}