Mail.dll - How to start

First you have to add reference to Mail.dll to your project. See MSDN how to. Then add all namespaces:

C# code:
using Lesnikowski.Client;
using Lesnikowski.Client.IMAP;
using Lesnikowski.Mail;
using Lesnikowski.Mail.Fluent
using Lesnikowski.Mail.Headers;
using Lesnikowski.Mail.Headers.Constants;

VB.NET code:
Imports Lesnikowski.Client
Imports Lesnikowski.Client.IMAP
Imports Lesnikowski.Mail
Imports Lesnikowski.Mail.Fluent
Imports Lesnikowski.Mail.Headers
Imports Lesnikowski.Mail.Headers.Constants

We connect to IMAP server, then log in and acquire IMail object using MailBuilder class.

C# code:

using(Imap imap = new Imap())
{
    imap.Connect("imap.server.com");       // or ConnectSSL for SSL
    imap.UseBestLogin("user", "password");

    imap.SelectInbox();
    List<long> uidList = imap.SearchFlag(Flag.Unseen);
    foreach (long uid in uidList)
    {
        IMail email = new MailBuilder()
            .CreateFromEml(imap.GetMessageByUID(uid));

        Console.WriteLine(email.Subject);
        Console.WriteLine(email.Text);
    }
    imap.Close();
}

VB.NET code:

Using imap As New Imap
    imap.Connect("imap.server.com")       ' or ConnectSSL for SSL
    imap.UseBestLogin("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.GetMessageByUID(uid))

        Console.WriteLine(email.Subject)
        Console.WriteLine(email.Text)
    Next
    imap.Close()
End Using

We connect to POP3 server, then log in and acquire IMail object using MailBuilder class.

C# code:

using(Pop3 pop3 = new Pop3())
{
    pop3.Connect("pop3.server.com");       // or ConnectSSL for SSL
    pop3.UseBestLogin("user", "password");

    foreach (string uid in pop3.GetAll())
    {
        IMail email = new MailBuilder()
            .CreateFromEml(pop3.GetMessageByUID(uid));

        Console.WriteLine(email.Subject);
        Console.WriteLine(email.Text);
    }
    pop3.Close();
}

VB.NET code:

Using pop3 As New Pop3
    pop3.Connect("pop3.server.com")       ' or ConnectSSL for SSL
    pop3.UseBestLogin("user", "password")

    For Each uid As String In pop3.GetAll()
       Dim email As IMail = New MailBuilder() _
            .CreateFromEml(pop3.GetMessageByUID(uid))

        Console.WriteLine(email.Subject)
        Console.WriteLine(email.Text)
    Next
    pop3.Close()
End Using

You have to create IMail object. Use MailBuilder class or fluent interface, then connect to your SMTP server and send your message.

C# code:

IMail email = Mail
    .Html(@"Html with an image: <img src=""cid:lena"" />")
    .AddVisual(@"c:\lena.jpeg").SetContentId("lena")
    .AddAttachment(@"c:\tmp.doc").SetFileName("document.doc")
    .To("to@mail.com")
    .From("from@mail.com")
    .Subject("Subject")
    .Create();

using(Smtp smtp = new Smtp())
{
    smtp.Connect("smtp.server.com");       // or ConnectSSL for SSL
    smtp.UseBestLogin("user", "password");

    smtp.SendMessage(email);                      

    smtp.Close();    
}                     

VB.NET code:

Dim email As IMail = Mail _
    .Html("Html with an image: <img src=""cid:lena"" />") _
    .AddVisual("C:\lena.jpeg").SetContentId("lena") _
    .AddAttachment("C:\tmp.doc").SetFileName("document.doc") _
    .To("to@mail.com") _
    .From("from@mail.com") _
    .Subject("Subject") _
    .Create()

Using smtp As Smtp = New Smtp
    smtp.Connect("smtp.server.com")       ' or ConnectSSL for SSL
    smtp.UseBestLogin("user", "password")

    smtp.SendMessage(email)

    smtp.Close()
End Using