Code for http://darb.io/blog/2012/02/24/50-sending-an-email-with-an-attachment-from-workflow-foundation/

normal.cs

// Set the variables
string smtpAddress = "mail.example.com";
string fromAddress = "from@example.com";

// Create the SMTPClient
var smtp = new SmtpClient(smtpAddress);
smtp.Credentials = new NetworkCredential("username", "password"); 

// Construct the MailMessage
var mail = new MailMessage();
mail.From = new MailAddress(fromAddress);
mail.To.Add("to@example.com");
mail.Subject = "Email subject here";
mail.Body = "Body";

// Attach the files
mail.Attachments.Add(new Attachment("filename"));
mail.Attachments.Add(new Attachment("filename"));

// Send the MailMessage
smtp.Send(mail);

sharepoint.cs

using (var web = new SPSite("http://localhost").OpenWeb())
{
    // Get the variables from SP
    string smtpAddress = web.Site.WebApplication.OutboundMailServiceInstance.Server.Address;
    string fromAddress = web.Site.WebApplication.OutboundMailSenderAddress;

    // Create the SMTPClient
    var smtp = new SmtpClient(smtpAddress);
    smtp.Credentials = CredentialCache.DefaultNetworkCredentials;

    // Construct the MailMessage
    var mail = new MailMessage();
    mail.From = new MailAddress(fromAddress);
    mail.To.Add("to@example.com");
    mail.Subject = "Email subject here";
    mail.Body = "Body";

    // Attach the files
    mail.Attachments.Add(new Attachment("filename"));
    mail.Attachments.Add(new Attachment("filename"));

    // Send the MailMessage
    smtp.Send(mail);
}