Adding a signature looks like this:

// C#

using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
{
   IMail email = Mail
       .Text("text")
       .From("alice@mail.com")
       .To("bob@mail.com")
       .Subject("subject")
       .DKIMSign(rsa, "brisbane", "example.com")
       .Create();
}

' VB.NET

Using rsa As New RSACryptoServiceProvider()
	Dim email As IMail = Mail _
		.Text("text") _
		.From("alice@mail.com") _
		.[To]("bob@mail.com") _
		.Subject("subject") _
		.DKIMSign(rsa, "brisbane", "example.com") _
		.Create()
End Using

So what you need is RSACryptoServiceProvider with your private key, and two strings: selector and domain.

Basically how this works is the recipient of the email queries the DNS server for TXT record for selector._domainkey.domain (in our sample it is: “brisbane._domainkey.example.com”) to get the public key and validate the signature.

You can use nslookup to get the public key for a domain:

Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\Pawel>nslookup
Default Server:  UnKnown
Address:  192.168.0.1

> set type=TXT
> gamma._domainkey.gmail.com
Server:  UnKnown
Address:  192.168.0.1

Non-authoritative answer:
gamma._domainkey.gmail.com      text =

        "k=rsa; t=y; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDIhyR3oItOy22ZOaBrI
Ve9m/iME3RqOJeasANSpg2YTHTYV+Xtp4xwf5gTjCmHQEMOs0qYu0FYiNQPQogJ2t0Mfx9zNu06rfRBD
jiIU9tpx2T+NGlWZ8qhbiLo5By8apJavLyqTLavyPSrvsx0B3YzC63T4Age2CDqZYA+OwSMWQIDAQAB"

gmail.com       nameserver = ns1.google.com
gmail.com       nameserver = ns4.google.com
gmail.com       nameserver = ns3.google.com
gmail.com       nameserver = ns2.google.com
ns1.google.com  internet address = 216.239.32.10
ns2.google.com  internet address = 216.239.34.10
ns3.google.com  internet address = 216.239.36.10
ns4.google.com  internet address = 216.239.38.10
>

But don’t worry as Mail.dll will make this DNS query for you.

Validating (including DNS query for public key) is simple:

// C#

IMail email = new MailBuilder()
    .CreateFromEmlFile("signed_gamma.gmail.eml");
bool isValid = email.CheckDKIMSignature();

' VB.NET

Dim email As IMail = New MailBuilder() _
    .CreateFromEmlFile("signed_gamma.gmail.eml")
Dim isValid As Boolean = email.CheckDKIMSignature()

You can download Mail.dll .NET email component here.

// C#

client.Connect("imap.example.com");

' VB.NET

client.Connect("imap.example.com");

Then, before logging-in, start implicit SSL negotiation. The command name differs for different protocols:

In case of IMAP:

// C#

client.StartTLS();

' VB.NET

client.StartTLS()

In case of POP3:

// C#

client.STLS();

' VB.NET

client.STLS()

In case of SMTP:

// C#

client.StartTLS();

' VB.NET

client.StartTLS()

Now, your connection is secured.

Remember that you can ignore SSL certificate errors using ServerCertificateValidate event:

// C#

client.ServerCertificateValidate +=
    (sender, e) => { e.IsValid = true; };

' VB.NET

client.ServerCertificateValidate += Function(sender, e) Do
	e.IsValid = True
End Function

Mail.dll .NET IMAP client allows you to download only needed parts of the specified message.

First you need to get the structure of the message.
There are two methods for that: GetBodyStructureByUID and GetBodyStructure.

BodyStructure contains information about plain text, html, and all attachments of the message. It does not contain any data though.

To download text parts of the email (like html or plain text) you can use: GetMimePartTextByUID or GetMimePartText.

To download attachments use: GetMimePartByUID or GetMimePart.

Here’s the full sample for this feature:

// C#

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

	imap.SelectInbox();
	List<long> uidList = imap.SearchFlag(Flag.Unseen);
	foreach (long uid in uidList)
	{
		// Get the structure of the email
		BodyStructure structure = imap.GetBodyStructureByUID(uid);

		// Download only text and html parts
		string text = imap.GetMimePartTextByUID(uid, structure.Text);
		string html = imap.GetMimePartTextByUID(uid, structure.Html);

		Console.WriteLine(text);
		Console.WriteLine(html);

		// Show all attachments' filenames
		foreach(MimeStructure attachment in structure.Attachments)
		{
			Console.WriteLine(attachment.FileName);
			// You can also download entire attachment
			byte[] bytes = imap.GetMimePartByUID(uid, attachment);
		}
	}
	imap.Close(true);
}

' VB.NET

Using imap As New Imap()
	imap.Connect("imap.server.com")
	imap.Login("user", "password")

	imap.SelectInbox()
	Dim uidList As List(Of Long) = imap.SearchFlag(Flag.Unseen)
	For Each uid As Long In uidList
		' Get the structure of the email
		Dim struct As BodyStructure = imap.GetBodyStructureByUID(uid)

		' Download only text and html parts
		Dim text As String = imap.GetMimePartTextByUID(
			uid, struct.Text)
		Dim html As String = imap.GetMimePartTextByUID(
			uid, struct.Html)

		Console.WriteLine(text)
		Console.WriteLine(html)

		' Show all attachments' filenames
		For Each attachment As MimeStructure In struct.Attachments
			Console.WriteLine(attachment.FileName)
			' You can also download entire attachment
			Dim bytes As Byte() = imap.GetMimePartByUID(
				uid, attachment)
		Next
	Next
	imap.Close(True)
End Using

If you don’t want this behavior, there are two things you can do.

First is to use Examine method instead of Select:

// C#

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

    imap.ExamineInbox(); // -or- imap.Examine("Inbox");

    // ...

    imap.Close(true);
}

' VB.NET

Using imap As New Imap
    imap.Connect("imap.server.com")
    imap.Login("user", "password")

    imap.SelectInbox()

    imap.ExamineInbox() ' -or- imap.Examine("Inbox")

    ' ...

    imap.Close(True)
End Using

Examine puts mailbox (also called folder) into read-only state, so no \Seen flag is added.
The drawback is that you can not delete a message.

Second approach is to use one of the Peek methods on the IMAP client:

// C#

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

    imap.SelectInbox();
    List<long> uidList = imap.SearchFlag(Flag.Unseen);
    foreach (long uid in uidList)
    {
        IMail email = new MailBuilder()
            .CreateFromEml(imap.PeekMessageByUID(uid));
        Console.WriteLine(email.Subject);
    }
    imap.Close(true);
}

' VB.NET

Using imap As New Imap
    imap.Connect("imap.server.com")
    imap.Login("user", "password")

    imap.SelectInbox()
    Dim uidList As List(Of Long) = imap.SearchFlag(Flag.Unseen)

    For Each uid As Long In uidList
        Dim email As IMail = New MailBuilder() _
            .CreateFromEml(imap.PeekMessageByUID(uid))
        Console.WriteLine(email.Subject)
    Next
    imap.Close(True)
End Using

There are Peek methods for downloading headers and even parts of the email message.

You can download Mail.dll IMAP client here.

However they require a specific, but non-standard IMAP command to be sent before login is done. The command is: “ID (“GUID” “1″)”

// C#
imap.SendCommand(@"ID (""GUID"" ""1"")");

' VB
imap.SendCommand("ID (""GUID"" ""1"")")

There is also an IMAP server running at imap-ssl.mail.yahoo.com; it uses SSL on the standard port 993.

Here’s the full C# version of the code:

using (Imap imap = new Imap())
{
    imap.Connect("imap.mail.yahoo.com");
    imap.SendCommand(@"ID (""GUID"" ""1"")");

	imap.Login("user", "password");

	imap.SelectInbox();
	List<long> uidList = imap.Search(Expression.All());
	foreach (long uid in uidList)
	{
		IMail email = new MailBuilder()
    			.CreateFromEml(imap.GetMessageByUID(uid));
		Console.WriteLine(email.Subject);
	}
	imap.Close(true);
}

and VB.NET version:

Using imap As New Imap()
	imap.Connect("imap.mail.yahoo.com")
	imap.SendCommand("ID (""GUID"" ""1"")")

	imap.Login("user", "password")

	imap.SelectInbox()
	Dim uidList As List(Of Long) = imap.Search(Expression.All())
	For Each uid As Long In uidList
		Dim email As IMail = New MailBuilder() _
			.CreateFromEml(imap.GetMessageByUID(uid))
		Console.WriteLine(email.Subject)
	Next
	imap.Close(True)
End Using

Differences:
- non-standard command ID command is required before any other command.
- Examine explicitly requires CLOSE command (Imap.CloseCurrentFolder), otherwise subsequent SELECT (Imap.Select) has no effect and mailbox is still in read-only state
- IDLE is not implemented.
- SELECT for not existing folder does not create new folder.

You can download the latest version of Mail.dll .NET IMAP client here

IMAP4 servers that support IDLE will include this programming string in the result of their CAPABILITY command.

You can use Capability method to check server’s capability list

// C# code

using (Imap client = new Imap())
{
	client.ConnectSSL("imap.server.com");
	client.Capability().ForEach(Console.WriteLine);
	client.Close();
}

' VB.NET code

Using client As New Imap()
	client.ConnectSSL("imap.server.com")
	client.Capability().ForEach(Console.WriteLine)
	client.Close()
End Using

Mail.dll IMAP client supports IDLE command.

There are two important methods:

• Idle() – starts accepting real-time notifications. The method hangs until a notification is received.
• StopIdle() – stops accepting real-time notifications.

// C# code

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

    FolderStatus folderStatus = client.SelectInbox();
    Console.WriteLine("Total message count: {0}",
        folderStatus.MessageCount);

    while(true)
    {
        FolderStatus currentStatus = client.Idle();
        Console.WriteLine("Total message count: {0}",
                currentStatus.MessageCount);
        foreach(long uid in client.SearchFlag(Flag.Unseen))
        {
            IMail email = new MailBuilder().CreateFromEml(
                client.GetHeadersByUID(uid));
            Console.WriteLine(email.Subject);
        }
    }
    client.Close();
}

' VB.NET code

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

	Dim folderStatus As FolderStatus = client.SelectInbox()
	Console.WriteLine("Total message count: {0}",_
		folderStatus.MessageCount)

	While True
		Dim currentStatus As FolderStatus = client.Idle()
		Console.WriteLine("Total message count: {0}",_
			currentStatus.MessageCount)
		For Each uid As Long In client.SearchFlag(Flag.Unseen)
			Dim email As IMail = New MailBuilder()_
 				.CreateFromEml(client.GetHeadersByUID(uid))
			Console.WriteLine(email.Subject)
		Next
	End While
	client.Close()
End Using

Now lets handle stop gracefully:

// C# code

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

    FolderStatus folderStatus = client.SelectInbox();
    Console.WriteLine("Total message count: {0}",
        currentStatus.MessageCount);

    bool stop = false;
    // We start a new thread to handle user input, <enter> = stop idle
    new Thread(() =>
	{
		Console.ReadLine();
		client.StopIdle();
		stop = true;
	}).Start();

    while(stop == false)
    {
        FolderStatus currentStatus = client.Idle();
        if (stop == true)
            break;

        Console.WriteLine("Total message count: {0}",
            currentStatus.MessageCount);

        foreach(long uid in client.SearchFlag(Flag.Unseen))
        {
            IMail email = new MailBuilder().CreateFromEml(
                client.GetHeadersByUID(uid));
            Console.WriteLine(email.Subject);
        }
    }
    client.Close();
}

' VB.NET code

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

	Dim folderStatus As FolderStatus = client.SelectInbox()
	Console.WriteLine("Total message count: {0}",_
		currentStatus.MessageCount)

	Dim [stop] As Boolean = False
	' We start a new thread to handle user input, <enter> = stop idle
	New Thread(Function() Do
		Console.ReadLine()
		client.StopIdle()
		[stop] = True
	End Function).Start()

	While [stop] = False
		Dim currentStatus As FolderStatus = client.Idle()
		If [stop] = True Then
			Exit While
		End If

		Console.WriteLine("Total message count: {0}",_
			currentStatus.MessageCount)

		For Each uid As Long In client.SearchFlag(Flag.Unseen)
			Dim email As IMail = New MailBuilder()_
				.CreateFromEml(client.GetHeadersByUID(uid))
			Console.WriteLine(email.Subject)
		Next
	End While
	client.Close()
End Using

1. We need to create an email message
2. Use src=”cid:imageID” as image source in HTML
3. Add the image to Visuals collection using AddVisual method
4. Apply image id by setting ContentId
5. And finally send the message

// Use builder class to create an email
MailBuilder builder = new MailBuilder();

// Set From, To
builder.From.Add(new MailBox("alice@mail.com", "Alice"));
builder.To.Add(new MailBox("bob@mail.com", "Bob"));
builder.Subject = "Test";

// Set HTML content (Notice the src="cid:..." attribute)
builder.SetHtmlData(@"<html><body><img src=""cid:image1"" /></body></html>");

// SetHtmlData automatically extracts plaint text from html so,
// unless you want to specify plain text explicitly, you don't need to use this method:
builder.SetTextData("This is text version of th message.");

// Add image and set its Content-Id
MimeData visual = builder.AddVisual(@"c:\image.jpg");
visual.ContentType = new ContentType(MimeType.Image, MimeSubtype.Jpeg);
visual.ContentId = "image1";

IMail email = builder.Create();

// Send the message
using (Smtp smtp = new Smtp())
{
    smtp.Connect("mail.host.com");
    smtp.Ehlo(HeloType.EhloHelo, "yourname");
    smtp.Login("user", "password");
    smtp.SendMessage(email);
    smtp.Close(false);
}

And of course the VB.NET version of the same code:

' Use builder class to create an email
Dim builder As New MailBuilder()

' Set From, To
builder.From.Add(New MailBox("alice@mail.com", "Alice"))
builder.To.Add(New MailBox("bob@mail.com", "Bob"))
builder.Subject = "Test"

' Set HTML content (Notice the src="cid:..." attribute)
builder.SetHtmlData("<html><body><img src=""cid:image1"" /></body></html>")

' SetHtmlData automatically extracts plaint text from html so,
' unless you want to specify plain text explicitly, you don't need to use this method:
builder.SetTextData("This is text version of th message.")

' Add image and set its Content-Id
Dim visual As MimeData = builder.AddVisual("c:\image.jpg")
visual.ContentType = New ContentType(MimeType.Image, MimeSubtype.Jpeg)
visual.ContentId = "image1"

Dim email As IMail = builder.Create()

' Send the message
Using smtp As New Smtp()
	smtp.Connect("mail.host.com")
	smtp.Ehlo(HeloType.EhloHelo, "yourname")
	smtp.Login("user", "password")
	smtp.SendMessage(email)
	smtp.Close(False)
End Using

Same code using fluent interface:

Mail.Html(@"<html><body><img src=""cid:image1"" /></body></html>")
        .Text("This is text version of th message.")
        .From(new MailBox("alice@mail.com", "Alice"))
        .To(new MailBox("bob@mail.com", "Bob"))
        .Subject("Test")
        .AddVisual(@"c:\image.jpg")
        .SetContentId("image1")
        .SetContentType(
                new ContentType(MimeType.Image, MimeSubtype.Jpeg))
        .UsingNewSmtp()
        .WithCredentials("lesnikowski","password")
        .Server("mail.host.com")
        .Send();

And of course the VB.NET version using fluent interface:

Mail.Html("<html><body><img src=""cid:image1"" /></body></html>") _
	.Text("This is text version of th message.") _
	.From(New MailBox("alice@mail.com", "Alice")) _
	.To(New MailBox("bob@mail.com", "Bob")) _
	.Subject("Test").AddVisual("c:\image.jpg") _
	.SetContentId("image1") _
	.SetContentType( _
                New ContentType(MimeType.Image, MimeSubtype.Jpeg)) _
	.UsingNewSmtp() _
	.WithCredentials("lesnikowski", "password") _
	.Server("mail.host.com") _
	.Send()

You can download Mail.dll email client here.