You need to use DeleteMessageByUID method. Messages will be still available in All Mail folder.

// C#

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

	// Find all emails we want to delete
	imap.SelectInbox();
	List<long> uids = imap.Search(
		Expression.Subject("email to archive"));

	imap.DeleteMessageByUID(uids);

	imap.Close();
}

' VB.NET

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

	' Find all emails we want to delete
	imap.SelectInbox()
	Dim uids As List(Of Long) = imap.Search(_
		Expression.Subject("email to archive"))

	imap.DeleteMessageByUID(uids)

	imap.Close()
End Using

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. SSelect 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 we need to use ConnectSSL method.

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 we need to use ConnectSSL method.

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. 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 Maill.dll and appropriate namespaces.

// C#

using Lesnikowski.Client.IMAP;
using Lesnikowski.Client.Authentication;
using Lesnikowski.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/");

Process.Start(url);
// You can use Response.Redirect(url) in ASP.NET

string oauthVerifier = HttpUtility.UrlDecode(Console.ReadLine());
// You can use Request["oauth_verifier"].ToString() in ASP.NET

oauth.GetAccessToken(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:

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 Lesnikowski.Client.IMAP
Imports Lesnikowski.Client.Authentication
Imports Lesnikowski.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/")

Process.Start(url)
' You can use Response.Redirect(url) in ASP.NET

Dim oauthVerifier As String = HttpUtility.UrlDecode(Console.ReadLine())
' You can use Request["oauth_verifier"].ToString() in ASP.NET

oauth.GetAccessToken(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 Maill.dll and appropriate namespaces.

// C#

using Lesnikowski.Client.IMAP;
using Lesnikowski.Client.Authentication;
using Lesnikowski.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(TestConstants.GmailImapServer);

    string oauthImapKey = oauth.GetXOAuthKeyForImap(email);

    client.LoginOAUTH(oauthImapKey);

    //...

    client.Close();
}

' VB.NET

Imports Lesnikowski.Client.IMAP
Imports Lesnikowski.Client.Authentication
Imports Lesnikowski.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(TestConstants.GmailImapServer)

	Dim oauthImapKey As String = oauth.GetXOAuthKeyForImap(email)

	client.LoginOAUTH(oauthImapKey)

	'...

	client.Close()
End Using

Here are the google apps configuration screens:

“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 point 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
    List<FolderInfo> allFolders = client.GetFolders();
    FolderInfo allMail = new CommonFolders(allFolders).AllMail;
    client.Select(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 allFolders As List(Of FolderInfo) = client.GetFolders()
	Dim allMail As FolderInfo = New CommonFolders(allFolders).AllMail
	client.[Select](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.

• Extension of the LIST command: XLIST
• Extension of the SEARCH command: X-GM-RAW
• Access to the Gmail unique message ID: X-GM-MSGID
• Access to the Gmail thread ID: X-GM-THRID
• Access to Gmail labels: X-GM-LABELS

You can read on how to use Mail.dll with Gmail in the following articles:

• Get Gmail thread id
• Search Gmail thread id
• Get Gmail message id
• Search Gmail message id
• Get Gmail labels for specified messages
• Label message with Gmail label
• Search Gmail label
• Localized Gmail IMAP Folders
• Search Gmail using Gmail’s search syntax
• Create Gmail url-ID via IMAP

As such, labels can be modified using the standard IMAP commands, CreateFolder, RenameFolder, and DeleteFolder, that act on folders.

System labels, which are labels created by Gmail, are reserved and prefixed by “[Gmail]” or “[GoogleMail]” in the list of labels.

Use the XLIST command to get the entire list of labels for a mailbox.

The labels for a given message may be retrieved by using the X-GM-LABELS attribute with the FETCH command.

It is also possible to use the X-GM-LABELS attribute to return the UIDs for all messages with a given label, using the SEARCH command.

// C# version

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

    imap.SelectInbox();

    List<long> uids = imap.Search().Where(
        Expression.GmailLabel("MyLabel"));

    foreach (MessageInfo info in imap.GetMessageInfoByUID(uids))
    {
        Console.WriteLine("{0} - {1}",
            info.Envelope.GmailThreadId,
            info.Envelope.Subject);
    }

    imap.Close();
}

' VB.NET version

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

	imap.SelectInbox()

	Dim uids As List(Of Long) = imap.Search().Where(Expression.GmailLabel("MyLabel"))

	For Each info As MessageInfo In imap.GetMessageInfoByUID(uids)
            Console.WriteLine("{0} - {1}",  _
                info.Envelope.GmailThreadId,  _
                info.Envelope.Subject)
	Next

	imap.Close()
End Using

You can learn more about this Gmail IMAP extension here:
http://code.google.com/apis/gmail/imap/#x-gm-labels

As such, labels can be modified using the standard IMAP commands, CreateFolder, RenameFolder, and DeleteFolder, that act on folders.

System labels, which are labels created by Gmail, are reserved and prefixed by “[Gmail]” or “[GoogleMail]” in the list of labels.

Use the XLIST command to get the entire list of labels for a mailbox.

The labels for a given message may be retrieved by using the X-GM-LABELS attribute with the FETCH command.

Labels may be added to a message using the STORE command in conjunction with the X-GM-LABELS attribute.

// C# version

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

    imap.SelectInbox();
    long uid = imap.GetAll().Last();

    imap.GmailLabelMessageByUID(uid, "MyLabel");

    List<string> labels = imap.GmailGetLabelsByUID(uid);
    labels.ForEach(Console.WriteLine);

    imap.Close();
}

' VB.NET version

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

	imap.SelectInbox()
	Dim uid As Long = imap.GetAll().Last()

	imap.GmailLabelMessageByUID(uid, "MyLabel")

	Dim labels As List(Of String) = imap.GmailGetLabelsByUID(uid)
	labels.ForEach(Console.WriteLine)

	imap.Close()
End Using

You can learn more about this Gmail IMAP extension here:
http://code.google.com/apis/gmail/imap/#x-gm-labels

As such, labels can be modified using the standard IMAP commands, CreateFolder, RenameFolder, and DeleteFolder, that act on folders.

System labels, which are labels created by Gmail, are reserved and prefixed by “[Gmail]” or “[GoogleMail]” in the list of labels.

Use the XLIST command to get the entire list of labels for a mailbox.

The labels for a given message may be retrieved by using the X-GM-LABELS attribute with the FETCH command.

For list of messages

// C# version

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

    imap.SelectInbox();

    List<long> uids = imap.GetAll();
    List<MessageInfo> infos = imap.GetMessageInfoByUID(uids);

    foreach (MessageInfo info in infos)
    {
        List<string> labels = info.GmailLabels.Labels;

        Console.WriteLine("[{0}] {1}",
            string.Join(" ", labels.ToArray()),
            info.Envelope.Subject);
    }

    imap.Close();
}

' VB.NET version

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

	imap.SelectInbox()

	Dim uids As List(Of Long) = imap.GetAll()
	Dim infos As List(Of MessageInfo) = imap.GetMessageInfoByUID(uids)

	For Each info As MessageInfo In infos
            Dim labels As List(Of String) = info.GmailLabels.Labels

	    Console.WriteLine("[{0}] {1}",  _
                String.Join(" ", labels.ToArray()),  _
                info.Envelope.Subject)
	Next

	imap.Close()
End Using

For single message:

// C# version

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

    imap.SelectInbox();

    long uid = imap.GetAll().Last();
    List<string> labels = imap.GmailGetLabelsByUID(uid);

    labels.ForEach(Console.WriteLine);

    imap.Close();
}

' VB.NET version

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

	imap.SelectInbox()

	Dim uid As Long = imap.GetAll().Last()
	Dim labels As List(Of String) = imap.GmailGetLabelsByUID(uid)

	labels.ForEach(Console.WriteLine)

	imap.Close()
End Using

You can learn more about this Gmail IMAP extension here:
http://code.google.com/apis/gmail/imap/#x-gm-labels