Frequently Asked Questions
SMTP account: Unable to authenticate with your username and password
Your mail account password may be wrong or empty (see Mail Accounts in Preferences).
If you are trying to send via your iCloud account with 2-factor authentication set up, you will need to set up an app-specific password for Mail Designer. The same may apply to other email providers with 2-factor authentication.
Apple's support page has more information on how to set up an app-specific password.