.Net System.Mail.Message adding multiple “To” addresses

I wasn’t able to replicate your bug:

var message = new MailMessage();

message.To.Add("user@example.com");
message.To.Add("user2@example.com");

message.From = new MailAddress("test@example.com");
message.Subject = "Test";
message.Body = "Test";

var client = new SmtpClient("localhost", 25);
client.Send(message);

Dumping the contents of the To: MailAddressCollection:

MailAddressCollection (2 items)
DisplayName User Host Address

user example.com user@example.com
user2 example.com user2@example.com

And the resulting e-mail as caught by smtp4dev:

Received: from mycomputername (mycomputername [127.0.0.1])
     by localhost (Eric Daugherty's C# Email Server)
     3/8/2010 12:50:28 PM
MIME-Version: 1.0
From: test@example.com
To: user@example.com, user2@example.com
Date: 8 Mar 2010 12:50:28 -0800
Subject: Test
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

Test

Are you sure there’s not some other issue going on with your code or SMTP server?