If you received the error:
Your username is not allowed to send from this domain, please see
https://help.mayfirst.org/en/help/email-domain-blocked
it means you will need to add the domain name of your "from" address to the control panel (e.g. if your address is joe@example.org, you would need to add example.org to the control panel).
To fix this problem email support with the username and email address you are sending from OR follow these instructsions to add an MX record.
Note: if you are not using May First as your DNS provider, you still need to add your domain name's MX record to demonstrate that you own the domain name.
This restriction (added in October 2024) ensures that no member can impersonate you by sending from your domain name.