Ring up your ISP again and let them know they have no business working on an email help desk. They d

Ring up your ISP again and let them know they have no business working on an email help desk. They do not have a clue how it works. When using an email client like Thunderbird you connect to POP or IMAP server to receive and an SMTP server to send. None of these servers are used when using the providers Webmail system. Trying to compare the two processes is meaningless.

From the error message you have the wrong name for the providers SMTP server. You need to get the correct server settings from your provider and make the necessary changes in Thunderbird. Since your provider configures their server maybe their help desk can come up with the correct settings.