You would really need to ask the provider if they are sending the mail. Their server logs will have information relevant such as if the remote server issued a warning or error and what it was. My guess is ttsco.net has charter blacklisted as a sender. Either through a blocklist such as spamhaus (which do not list it) or one of the many others. Is the domain name of the sending email a charter domain? Most servers these days use DNS records for permitted senders for a domain, so the old process encouraged by US ISP's of sending ALL mail through their servers fails often these days. Can the server admin at ttsco locate the messages in there mail server spam folder or if it is an onsite setup do they have an edge device that checks mail for spam and simply drops it when it is found? Mail has exactly no guarantee of delivery, whilst most mail get delivered in seconds, there is no obligation on a mail server to even accept mail from certain others and they regularly drop mail that is considered spam. So instead of asking strangers why your mail is not completing it's journey, ask those that have the power to stop it what is happening with it. All Thunderbird knows is your providers server was happy to accept the mail for delivery. (You would have received an error on sending if it was not). Beyond that point it is the mail provider and the receiving server that can offer information that will help you in tracking the issue.