To enable POP3 in Gmail:

1. Sign in to Gmail.
2. Click the gear icon in the upper-right and select Mail settings .
3. Click Forwarding and POP/IMAP.
4. Select Enable POP for all mail or Enable POP for mail that arrives from now on.
5. Now you are able to connect to your Gmail account with Mail.dll POP3 library.

Remember that Gmail only allows secure SSL connections so you need to use ConnectSSL method.

Now you can download emails from Gmail using POP3 protocol.

To enable IMAP in Gmail:

1. Sign in to Gmail.
2. Click the gear icon in the upper-right and select Mail settings .
3. Click Forwarding and POP/IMAP.
4. Select Enable IMAP.
5. Now you are able to connect to your Gmail account with Mail.dll IMAP library.

Remember that Gmail only allows secure SSL connections so you need to use ConnectSSL method.

Now you can download emails from Gmail using IMAP protocol.

OAuth is an open protocol to allow secure API authorization in a simple and standard method from desktop and web applications.

In this post I’ll show how to access Gmail account using 3-legged OAuth authentication method with Mail.dll IMAP component. The key advantage of this method is that it allows an application to access user email without knowing user’s password.

You can read more on OAuth authentication with Google accounts here:
http://code.google.com/apis/accounts/docs/OAuth_ref.html

Gmail IMAP and SMTP using OAuth:
http://code.google.com/apis/gmail/oauth/protocol.html

If your application/website is not registered, you should use following key and secret:
consumer key: “anonymous”
consumer secret: “anonymous”

Remember to add reference to Mail.dll and appropriate namespaces.

// C#

using Limilabs.Client.IMAP;
using Limilabs.Client.Authentication;
using Limilabs.Client.Authentication.Google;

const string userEmailAccount = "pat@gmail.com";
const string consumerKey = "anonymous";
const string consumerSecret = "anonymous";

GmailOAuth oauth = new GmailOAuth(consumerKey, consumerSecret);

string url = oauth.GetAuthorizationUrl("http://localhost:64119/");

// ASP.NET client:
// Save oauth in permanent storage:
// Cache[oauth.RequestToken.Token] = oauth;

// Windows client:
Process.Start(url);

// ASP.NET client:
// Response.Redirect(url);

// Windows client with url:
string rawReturnUrl = Console.ReadLine();
ReturnUrl returnUrl = new ReturnUrl(rawReturnUrl);
oauth.GetAccessToken(returnUrl.OAuthVerifier);

// Windows client with verification code (oob):
// string oauthVerifier = HttpUtility.UrlDecode(Console.ReadLine());
// oauth.GetAccessToken(oauthVerifier);

// ASP.NET client:
// ReturnUrl returnUrl = new ReturnUrl(Request.RawUrl);
// Retrieve oauth from permanent storage:
// GmailOAuth oauth = Cache[returnUrl.OAuthToken]
// oauth.GetAccessToken(returnUrl.OAuthVerifier);

using (Imap client = new Imap())
{
    client.ConnectSSL("imap.gmail.com");
    string oauthImapKey = oauth.GetXOAuthKeyForImap();
    client.LoginOAUTH(oauthImapKey);

    // Now you can access user's emails
    //...

    client.Close();
    oauth.RevokeToken(oauthImapKey);
}

1.
GmailOAuth.GetAuthorizationUrl method returns url you should redirect your user to, so he can authorize access.
As you can see, Mail.dll is asking for access to user’s email information and Gmail access:

2.
If you don’t specify callback parameter, user will have to manually copy&paste the token to your application (oob):

In case of a web project, you can specify a web address on your website. oauth_verifier will be included as the redirection url parameter.

After the redirection, your website/application needs to read oauth_verifier query parameter:

3.
GmailOAuth.GetAccessToken method authorizes the token.

4.
GmailOAuth.GetXOAuthKeyForImap method uses Google API to get the email address of the user, and generates XOAuth key for IMAP protocol (you can use GetXOAuthKeyForSmtp for SMTP).

5.
GmailOAuth.RevokeToken method revokes XOAuth key, so no further access can be made with it.

…and finally VB.NET version of the code:

' VB.NET

Imports Limilabs.Client.IMAP
Imports Limilabs.Client.Authentication
Imports Limilabs.Client.Authentication.Google

Const  userEmailAccount As String = "pat@gmail.com"
Const  consumerKey As String = "anonymous"
Const  consumerSecret As String = "anonymous"

Dim oauth As New GmailOAuth(consumerKey, consumerSecret)

Dim url As String = oauth.GetAuthorizationUrl("http://localhost:64119/")

' ASP.NET client:
' Save oauth in permanent storage:
' Cache[oauth.RequestToken.Token] = oauth;

' Windows client:
Process.Start(url)

' ASP.NET client:
' Response.Redirect(url)

' Windows client with url:
Dim rawReturnUrl As String = Console.ReadLine()
Dim returnUrl As New ReturnUrl(rawReturnUrl)
oauth.GetAccessToken(returnUrl.OAuthVerifier)

' Windows client with verification code (oob):
' Dim oauthVerifier As String = HttpUtility.UrlDecode(Console.ReadLine())
' oauth.GetAccessToken(oauthVerifier)

' ASP.NET client:
' Dim returnUrl As New ReturnUrl(Request.RawUrl)
' Retrive oauth from permanent storage:
' Dim oauth As GmailOAuth = Cache(returnUrl.OAuthToken)
' oauth.GetAccessToken(returnUrl.OAuthVerifier)

Using client As New Imap()
	client.ConnectSSL("imap.gmail.com")
	Dim oauthImapKey As String = oauth.GetXOAuthKeyForImap()
	client.LoginOAUTH(oauthImapKey)

	' Now you can access user's emails
	'...

	client.Close()
	oauth.RevokeToken(oauthImapKey)
End Using

OAuth is an open protocol to allow secure API authorization in a simple and standard method from desktop and web applications.

In this post I’ll show how to access Gmail account using 2-legged OAuth authentication method. The basic idea is that domain administrator can use this method to access user email without knowing user’s password.

You can read more on OAuth authentication with Google accounts here:
http://code.google.com/apis/accounts/docs/OAuth_ref.html

Gmail IMAP and SMTP using OAuth:
http://code.google.com/apis/gmail/oauth/protocol.html

Remember to add reference to Mail.dll and appropriate namespaces.

// C#

using Limilabs.Client.IMAP;
using Limilabs.Client.Authentication;
using Limilabs.Client.Authentication.Google;

const string consumerKey = "example.com";
const string consumerSecret = "secret";
const string email = "pat@example.com";

Gmail2LeggedOAuth oauth = new Gmail2LeggedOAuth(
    consumerKey, consumerSecret);

using (Imap client = new Imap())
{
    client.ConnectSSL("imap.gmail.com");

    string oauthImapKey = oauth.GetXOAuthKeyForImap(email);

    client.LoginOAUTH(oauthImapKey);

    //...

    client.Close();
}

' VB.NET

Imports Limilabs.Client.IMAP
Imports Limilabs.Client.Authentication
Imports Limilabs.Client.Authentication.Google

Const  consumerKey As String = "example.com"
Const  consumerSecret As String = "secret"
Const  email As String = "pat@example.com"

Dim oauth As New Gmail2LeggedOAuth(consumerKey, consumerSecret)

Using client As New Imap()
	client.ConnectSSL("imap.gmail.com")

	Dim oauthImapKey As String = oauth.GetXOAuthKeyForImap(email)

	client.LoginOAUTH(oauthImapKey)

	'...

	client.Close()
End Using

Here are the google apps configuration screens:

This is Gmail link that points to certain conversation (entire thread):

https://mail.google.com/mail/u/0/#inbox/13216515baefe747

“13216515baefe747″ is the Gmail thread-ID in hex.

Here’s the code that:

1. Selects “All Mail” folder
2. Gets the newest message UID
3. Obtains Gmail thread ID for this message (X-GM-THRID)
4. Converts it to hex
5. Creates the url that points to the Gmail conversation

// C# version

using (Imap client = new Imap())
{
    client.ConnectSSL("imap.gmail.com");
    client.Login("pat@gmail.com", "password");

    // Select 'All Mail' folder
    CommonFolders common = new CommonFolders(client.GetFolders());
    client.Select(common.AllMail);

    // get IMAP uid of the newest message
    long lastUid = client.GetAll().Last();

    // get message info
    MessageInfo info = client.GetMessageInfoByUID(lastUid);

    // extract Gmail thread ID
    Int64 threadId = Int64.Parse(info.Envelope.GmailThreadId);
    string threadIdAsHex = threadId.ToString("X");

    // create url
    string url = string.Format(
        "https://mail.google.com/mail/u/0/#inbox/{0}",
        threadIdAsHex);

    Console.WriteLine(url);

    client.Close();
}

' VB.NET version

Using client As New Imap()
	client.ConnectSSL("imap.gmail.com")
	client.Login("pat@gmail.com", "password")

	' Select 'All Mail' folder
	Dim common As New CommonFolders(client.GetFolders())
	client.Select(common.AllMail)

	' get IMAP uid of the newest message
	Dim lastUid As Long = client.GetAll().Last()

	' get message info
	Dim info As MessageInfo = client.GetMessageInfoByUID(lastUid)

	' extract Gmail thread ID
	Dim threadId As Int64 = Int64.Parse(info.Envelope.GmailThreadId)
	Dim threadIdAsHex As String = threadId.ToString("X")

	' create url
	Dim url As String = String.Format( _
		"https://mail.google.com/mail/u/0/#inbox/{0}", _
		threadIdAsHex)

	Console.WriteLine(url)

	client.Close()
End Using

Here you can find some more information about how to search by X-GM-THRID and all other Gmail IMAP extensions.