Getting SmtpClient to work with a self signed SSL certificate

You may take a look at the ServerCertificateValidationCallback property:

ServicePointManager.ServerCertificateValidationCallback = 
    (sender, certificate, chain, sslPolicyErrors) => true;

It represents a callback which is called by the runtime when it tries to validate an SSL certificate. By returning true you basically say that you don't care if the certificate is valid or not -> you always accept it. Of course having self signed certificates in production environment is not a good idea.

My issue ended up being that the .Net SmtpClient class apparently doesn't support the use of port 465 for SMTP SSL connections. Using port 25 with a self signed SSL certificate worked correctly.

MSDN System.Net forum question Can SmtpClient be configured to work with a self signed certificate?.