In the Control Panel/Domains, I have set up some domains as aliases for my main domain. Only my main domain has mail hosting. The other domains are verified as aliases for the main domain, and emails sent to <user>@<alias domain> arrive successfully at the main mail accounts.

I would like to be able to send emails with the From address as <user>@<alias domain> but the only way I found so far is to set up as an external email address, which results in a "Sent By <user@main domain>" header/message in the emails.

How can I send emails from <user>@<alias domain> without the "Sent By" message?