Apple Mail (nor Thunderbird mail) cannot send

cannot send mail but can receive mail

This is a message I get on Apple Mail:

"Could not connect to this SMTP server. Check your network connection and that you entered the correct information in the Account preferences. Also, verify that the server supports SSL. If it does not, deselect the "Use TLS/SSL" checkbox in the Server Setting tab of Account preferences"

Everything checked out OK that I could see on the Accounts/Server settings page: the host name, user name, password the port numbers. Except under Outgoing Mail Server (SMTP) Account says "Offline." All the other boxes are filled out correctly (user name, password, host name, port number).

The message I get when trying to use Thunderbird mail:

"Send Message Error

Sending of the message failed.

An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: [C18] RBL restriction: Blacklisted by Internal Reputation Service - 66.119.222.230."

What does any of this mean, and how do I correct it?

Thank you.