When I try to use the PHP library Swiftmailer to send an email I get an error saying that relaying is disallowed. I want to use a single user's credentials (the admin user, btw) to send email for any email address on the same domain. It will be impractical for me to require all of my users to supply their password every time they want to send an email, and unsafe for me to store those passwords.
I see that I can configure each of my users as a From address that I can use with my admin account. But this too would be impractical. And I'm not sure it would fix the problem, anyway.
It seems ridiculous to me that the extremely common use case of sending messages through an email script hasn't been addressed. I hope I've just missed the instructions.