best practice to implement in-application mailing/inbox system

I need to implement an in-application message-box system (like any forum/social network has).

Number of users is in the tens of thousands and up.

I am thinking to do it using an actual email server and write a client for it inside the application (or just pick an 开发者_如何学编程open source client and adjust it to appear like a private messages system).

Or

Should I just do it as an integral part of my application (DB, server side and all)?

Which approach is the correct one? (I do not think this is a consideration, but the bulk of the application is written in PHP).

Use your application back-end.

Email (SMTP/IMAP) sounds simple but has way too much administration and programming overhead if you do it properly - or you get random problems.

If you need to send emails to your users (which you shouldn't need to - I quite like how Stackoverflow works for example in that respect) then just be a client to email (SMTP).

Another issue with 'email' is it's an old protocol which does not fit into todays needs. Hence we have new protocols and applications, like XMPP and Google Wave.