Mailkit: Allowing only secure connections with SecureSocketOptions.Auto?

[rhagh]

I am trying to replace the framework internal SmtpClient in my application with MailKit's SmtpClient and encountered a problem when connecting to an insecure (i.e. no SSL/TLS) mail server using SecureSocketOptions.Auto. This results in connection being established insecure.

In the framework SmtpClient when setting option EnableSsl = true, this scenario leads to a SmtpException: Server does not support secure connections, and connection is not established.

I expected SecureSocketOptions.Auto would work the same, i.e. establish only a secure connection and just decide whether a TLS wrapped or STARTTLS upgraded connection would be used.

Is it possible to achieve this somehow?

[jstedfast]

The equivalent of EnableSsl from System.Net.Mail's SmtpClient would be SecureSocketOptions.StartTls.

Something a bit closer to what you want to do could be done like this:

var options = port == 465 ? SecureSocketOptions.SslOnConnect : SecureSocketOptions.StartTls`

That's very similar to what MailKit's internal logic is (except that it uses SecureSocketOptions.StartTlsWhenAvailable).

Hope that helps.

[adamfk]

So if I understand correctly, SecureSocketOptions.Auto may initiate an insecure connection if the server doesn't support it?

Thanks for the wonderful library!

[jstedfast]

Correct. If the server doesn't support StartTLS and the port is not the SSL-wrapped port (aka 465), then you will end up with an unencrypted connection.

[adamfk]

Would you mind if I made a PR to add your above description to the documentation for SecureSocketOptions.Auto.

I initially had the wrong impression reading: Allow the MailKit.IMailService to decide which SSL or TLS options to use (default).

[jstedfast]

No need, I just updated the docs myself.