My guess is you have more than one email address and are using the same SMTP server for both. verify by going to account settings, scroll down left side to 'Outgoing server (SMTP) and view the SMTP servers. There must be one for each account and have the email address specified and a unique Description field for each. Once you do that, revisit the account pane for each and adjust the bottom line that shows SMTP server and verify that it points to the one for that account.

To clarify, you are sending from email address A and using the SMTP server for email address B.

I agree that's a good guess, but I only have one email and only that one choice for the outgoing server.

Okay, look at your account email and your smtp server email. the error message is indicating they are not the same. My post was because this usually happens when a person has several email accounts. OR it could mean that authentication is required and you are not using it. If unsure, please post a screenshot of the SMTP server pane. Thank you.

The account and smtp server emails match. No authentication is required. I have not changed anything on my end and the issue just started for no apparent reason - maybe the problem is with my internet provider? But I can send email through that account on their webmail and my iphone.

I suggest authenticating. Your email provider wants more than you're providing. You might get more specific suggestions if you post a screenshot of account email settings and SMTP settings.