Hi zipstarman,
Here's something from the Mac user guide that might be helpful:
"In the Mail app on your Mac, if the Send button is dimmed in the toolbar of your message, or your message can’t be sent using the outgoing Mail server, try these suggestions.
- Make sure the outgoing (SMTP) mail server is specified as directed by the provider of your email account. See Verify an email account’s outgoing server.
- Choose Window > Connection Doctor, then follow the instructions displayed in the Details column.
- If there’s a firewall problem, verify that firewall software isn’t blocking email traffic on port 25, 465, or 587. Also check whether you’re affected by another firewall, such as software installed separately from macOS or built into an internet sharing router, or administered by your email account provider. Contact the appropriate person for information."
You can check out the full resource here: If you can’t send emails in Mail on Mac - Apple Support
If things persist you can reach out for additional support here: Get Support
Take care.