You could try sending an email with 456 bcc's in it.
Thought about that already and didn't want to do it because I want each recipient to receive his/her 'own' mail.
I've considered quite many possibilities, and in order to compare them successfully I would really need to know if it's possible to use bulk_mailer more or less directly from php.
I know how to do the trick with sendmail+bulk_mailer (example given in the readme of bulk_mailer), but that would require editing /ect/aliases, which is also something I want to avoid.
Oh, and in case you didn't know, bulk_mailer can be downloaded at ftp://cs.utk.edu/pub/moore/bulk_mailer/