Sending email via PHP fails, MX records point elsewhere

If you do not use our email service but use Gmail, for example, and point your MX records there, you also need to change an extra setting.

In DirectAdmin, under 'E-mail Manager', click MX Records and untick the following checkbox: 'Use this server to handle my e-mails. If not, change the MX records and uncheck this option.'

If you do not do this, the server itself does not know that it should deliver the email elsewhere, and you may get this message, among others:
Warning: mail() [function.mail]: SMTP server response: 550 Requested action not taken: mailbox unavailable or not local

Ideally, if you use a mail server elsewhere, you should also send email via that mail server, in other words via SMTP.