Provide MailboxAddress accessors for LocalPart and Domain.

Fixes issue #766

MimeKit/MailboxAddress.cs

@@ -238,6 +238,30 @@ public string Address {
 			}
 		}
 
+		/// <summary>
+		/// Get the local-part of the email address.
+		/// </summary>
+		/// <remarks>
+		/// Gets the local-part of the email address, sometimes referred to as the "user" portion of an email address.
+		/// </remarks>
+		public string LocalPart {
+			get {
+				return at != -1 ? address.Substring (0, at) : address;
+			}
+		}
+
+		/// <summary>
+		/// Get the domain of the email address.
+		/// </summary>
+		/// <remarks>
+		/// Gets the domain of the email address.
+		/// </remarks>
+		public string Domain {
+			get {
+				return at != -1 ? address.Substring (at + 1) : string.Empty;
+			}
+		}
+
 		/// <summary>
 		/// Get whether or not the address is an international address.
 		/// </summary>

UnitTests/MailboxAddressTests.cs

@@ -95,6 +95,20 @@ public void ArgumentExceptionTests ()
 			Assert.DoesNotThrow (() => new SecureMailboxAddress (Encoding.UTF8, "Routed Address", new[] { "route1", "route2", "route3" }, "[email protected]", "ffff"));
 		}
 
+		[Test]
+		public void TestLocalPartAndDomain ()
+		{
+			var mailbox = new MailboxAddress ("User Name", "[email protected]");
+
+			Assert.AreEqual ("user", mailbox.LocalPart, "LocalPart");
+			Assert.AreEqual ("domain.com", mailbox.Domain, "Domain");
+
+			mailbox = new MailboxAddress ("User Name", "user");
+
+			Assert.AreEqual ("user", mailbox.LocalPart, "Unix LocalPart");
+			Assert.AreEqual (string.Empty, mailbox.Domain, "Unix non-Domain");
+		}
+
 		[Test]
 		public void TestSetEmptyAddress ()
 		{