Webserver email queue
I run a webserver that hosts a ton of websites and they are getting busier. I don't run my own mail server and instead have PHP configured to send email via my ISPs SMTP server.
I'm now facing a problem where some of these websites send out email too fast and my ISPs server is dropping the messages because I have reached a threshhold.
I'd like to set up an SMTP "relay" daemon locally that the webserver will send the emails to and then this relay daemon will send the emails out at a steady pace. This will hopefully defeat the problem, although delaying emails at peak times which is totally acceptable.
What's the best way to set this up? Any suggestions on mailservers/smtp agents? I'm only asking because I've seen quite a few of them and they all advertise themselves as being "the best" for the job... which doesn't help me! So please, speak from experience, as I have none.