Dns Test Fail On Correct Dns Asp.net

I do an dns test before i send mail to ensure i dont get errors.But there is a problem, when i try to send an email to an adress xxxxx@eco-log.sei get an error from the dns function. when i try to delete the "-" in the mail adress it works.at first i tohught it was because of the "-", but i made an email at zzzzzz@eh-design.se and it works like a charm, what could be the problem then? It is 100% sure that "xxxxx@eco-log.se" is an existing email/dns.The error message im getting is "No such known source"(freely translated)

Heres the dns check code:

Public Function testDNS(ByVal dnsstring As String) As Boolean
Dim email As String = dnsstring
Dim host As String() = email.Split("@")
Dim hostName As String = host(1)

[code]....