"No sent mailbox configured. Please pick one in the account settings." - Mail app not able to send email

I have two NC accounts: account A (admin), and account B (non-admin)

I’m able to send and receive email through the mail app for my account A using A’s specific email (a@email.com)

I’m able to receive but NOT send through the mail app for account B using B’s specific email (b@email.com)

The error the app gives is: Error sending your message No sent mailbox configured. Please pick one in the account settings.

In the admin logs, it says: Error mail Saving draft failed: No drafts mailbox configured

Account B has the same email (b@email.com) registered in its NC account user email field.

What is going on? How can I fix this issue?

Thanks!