If you want to use user-defined label please read label message with user defined label.

There are two ways of labeling message with Gmail system label (e.g. Starred)

Imap.GmailLabelMessageByUID method

GmailLabelMessageByUID method uses Gmail’s X-GM-LABELS extension to the IMAP protocol. You need to provide a folder flag name for it to work correctly e.g. @”\Starred”.

You can use FolderFlag class static properties to get common flags:
FolderFlag.XImportant, FolderFlag.XSpam, FolderFlag.XStarred.

// C#

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

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

    imap.GmailLabelMessageByUID(last, FolderFlag.XStarred.Name);

    imap.Close();
}

' VB.NET

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

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

    imap.GmailLabelMessageByUID(last, FolderFlag.XStarred.Name)

    imap.Close()
End Using

Imap.CopyByUID method

The second method basis on the fact that Gmail treats labels as regular IMAP folders.
You just need to know correct folder name and copy the message to this folder.

You can use CommonFolders class to get common folders:

// C#

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

    List<FolderInfo> folders = imap.GetFolders();

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

    imap.CopyByUID(last, new CommonFolders(folders).Flagged);

    imap.Close();
}

' VB.NET

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

    Dim folders As List(Of FolderInfo) = imap.GetFolders()

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

    imap.CopyByUID(last, New CommonFolders(folders).Flagged)

    imap.Close()
End Using

Gmail treats labels as folders for the purposes of 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.

// C#

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

    List<FolderInfo> folders = imap.GetFolders();
    List<FolderInfo> system = folders.FindAll(x => x.Name.StartsWith("[Gmail]"));
    List<FolderInfo> user = folders.FindAll(x => !x.Name.StartsWith("[Gmail]"));

    Console.WriteLine("System labels:");
    system.ForEach(x => Console.WriteLine(x.Name));
    Console.WriteLine();
    Console.WriteLine("User labels:");
    user.ForEach(x => Console.WriteLine(x.Name));

    imap.Close();
}

' VB.NET

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

    Dim folders As List(Of FolderInfo) = imap.GetFolders()
    Dim system As List(Of FolderInfo) = 
        folders.FindAll(Function(x) x.Name.StartsWith("[Gmail]"))
    Dim user As List(Of FolderInfo) = 
        folders.FindAll(Function(x) Not x.Name.StartsWith("[Gmail]"))

    Console.WriteLine("System labels:")
    system.ForEach(Function(x) Console.WriteLine(x.Name))
    Console.WriteLine()
    Console.WriteLine("User labels:")
    user.ForEach(Function(x) Console.WriteLine(x.Name))

    imap.Close()
End Using

Here is the output:


System labels:
[Gmail]
[Gmail]/All Mail
[Gmail]/Drafts
[Gmail]/Sent Mail
[Gmail]/Spam
[Gmail]/Starred
[Gmail]/Trash



User labels:
Inbox
Personal
Receipts
Travel
Upload
Work
my label
my label/nested
my second label


Here’s the list of Gmail IMAP protocol extensions implemented in Mail.dll:

• 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
• OAuth and OAuth 2.0: XOAUTH, XOAUTH2

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

OAuth 2.0

• OAuth 2.0 with Gmail over IMAP for web applications
• OAuth 2.0 with Gmail over IMAP for installed applications

OAuth 1.0

• OAuth with Gmail
• 2-legged OAuth with Gmail
• Get Google contacts

Gmail treats labels as folders for the purposes of 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.UseBestLogin("pat@gmail.com", "password");

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

    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.UseBestLogin("pat@gmail.com", "password")

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

	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

If you want to use system label, such as \Starred or \Important please read label message with system label.

Gmail treats labels as folders for the purposes of 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.

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.UseBestLogin("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.UseBestLogin("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