<SERVER> Service not available, closing transmission channel
This is an unfortunately, open ended error. Most commonly this is in relation to the destination server not being "reachable", however it may also mean that your sending mail server is not "reachable". The latter of the two options is less common, unless a remote MX exchange is being used. The error message is also an indication that the message will continue being attempted for delivery.
If you are using a remote mail server for sending email, you'll want to ensure that it's functioning properly and connections to it are successful.
If you are not using a remote mail server (or it's working properly) you'll want to check the destination server's availibilty. Check out the Testing Remote SMTP / Mail Server Connections article referenced at the end of this article.