Send mail over smtp throws serval exceptions

Hey guys, I have made a piece of code in c# to simplify the process of sending an email over smtp. First, here ist the snippet for sending:

and here is the SendCompleteCallback:

If I then try to send an email, serval errors occur: Sometimes I get an

Mail System.Net.Mail.SmtpException: Message could not be sent. ---> System.IO.IOException: Connection closed

the other time its an

System.Net.Mail.SmtpFailedRecipientsException failed recipients

On some sites they say, you have to install the correct certificates, but the commands the use are only working on linux and I'm running windows 8. I'm also not using gmail, but an mailadress provided by a webhost, port and host are correct, and the credentials to.

Maybe you guys know a solution.

Greetings Hendrik Müller