Mac Mail asking for password for defunct account

I'm working on a 2017 era MacBook, running Ventura (13.6.4), using Apple's Mail program. Recently, whenever I try to send an email with attachment using my .mac (i.e. icloud) account, I get an error message asking for the password for a completely different email account: one that I no longer have access to.

Here's a screenshot:

The email its asking about is from a previous job. When working that job, I used this computer and Mail to access that email. But when I left the job, I removed the email from Mail, and from the System Settings>Accounts section. That was nearly 2 years ago, and this issue only cropped up a few days ago.

My .mac/icloud account long predates the work email. I can send mail with attachments using other email addresses. If I sign into iCloud in a browser, I can send emails with attachments using my .mac address. But not from my Mac in mail.

I found this thread, and tried the suggestion there (removing all traces of the old email from keychain access) but am still having this issue.