Hi there annie497,
Thank you for posting to Apple Support Communites.
We understand you're receiving an error message when trying to send mail. It's a good article you referenced with If you can’t send or receive email on your Mac
Let's have your rebuild the affected mailbox: Rebuild mailboxes in Mail on Mac
- In the Mail app
on your Mac, select the mailbox(s) listed in the sidebar you wish to rebuild. Hold shift key down over mailbox names to select more than one if desired. - Then go to top of the Mail app option (at TOP of screen not sidebar) and choose Mailbox, then select the option > Rebuild. This will rebuild the mailboxes you have selected in the sidebar
That should help. Let us know if you have any questions.
Take care.