We want to shift out our outgoing emails onto a separate server. We are running a social networking site and so we generate a good amount of mails, but not huge. So we want to prepare from now. Our main application server is a normal LAMP (PHP) stack. An other server would be used for email. This is what I mainly have/need:

PHP codes generate mails (all text emails) and saves to a MySQL of file or similar on the mail server. like a mail queue.
The mail server would have a cron which will check the mail queue and send out the actual mails. The cron could run once every 10 minutes or so.

There could be better solution... please suggest.
Now what software stack is best for the mail server? Postfix? The application server should remain untouched from software/hardware point of view if possible.