Such problems can be challenging. I suggest you try sending the same message - every word - to yourself. That would possibly let you see from where the message is coming from, your server detecting the error or the recipient's. Possible problem could be your IP address for thunderbird (which would be different from webmail), and any possible attachments you are sending (such as an embedded ad from an antivirus product). Or it may be an error that your sending email address does not match your SMTP email address. Can you send messages successfully to others?

I can send email to everyone else in my contacts list (at least so far). I think you may be on to something though. This a reply to a long email chain that has some links embedded in the body of the email. Some of the senders and recipients within the body of the email are unknow to me. I'm going to try to send a clean email to my client to see if that solves the problem. Thank you!