Recently i have been working on a popular web app that currently has around 70k members and a lot of traffic, i migrated the site to a new extra large EC2 instance and it seems to be a massive improvement, the site is a lot faster now and all images are stored on S3 to lessen the burden for the instance...
I have found that the bottleneck is now emails, the app currently sends a lot of emails, notifications on activity from users mostly, similar to the ones you get from Facebook when activity has occurred but their are reports of some users not getting emails such as verification when signing up etc and also we hit the SMTP limit on EC2 within less than a day of being on the new server proper (after DNS had resolved and users began using the site again)
I am just wondering what is the next step i could take scalability wise, the site is currently just using the built in php mail function, what can i do to ensure no problems occur with the site and the emails being delivered, would a dedicated email server be the next step ? how would that integrate with the current site ?
Any help that you could offer would be very useful...