Since this Thunderbird email client is my normal client for sending email, I simply kept (reused) the external SMTP server configuration (my ISP's).
Now, when I try to send email from one internal (example.com) account to another, I get the following error:
An error occurred while sending mail. The mail server responded: 5.1.2 Bad destination system: firstname.lastname@example.org. Please check the message recipients and try again.
What does this problem mean and how I fix it?
Is it possible that for testing Mercury accounts, I need to use Mercury's SMTP?