[.Net 2.0] Mailmessage only sends to 1 receiptent

[Remc0]

I'm trying to send an email from a website with mailmessage. It works all fine, but... Only 1 recipient will get the message. Other recipients or cc's won't get delivered. The recipient that gets the email can see in the header that the message is send to multiple recipients and cc's. I'm sure the email adresses are correct. I tried sending them in the same domain, other domains etc... When I send the message to a file, it also looks OK. There are also no errors thrown. What am I doing wrong ?

Const ToAddress As String = "remco@a.nl"
Const fromaddress As String = "test@a.nl"

Dim mm As New MailMessage(fromaddress, ToAddress)
mm.CC.Add(New MailAddress("corinda@a.nl"))
mm.To.Add(New MailAddress("corinda@a.nl", "Jan A"))

mm.Subject = "onderwerp"
mm.Body = "Inhoud"
mm.IsBodyHtml = False

Dim smtp As New SmtpClient
smtp.Send(mm)