My company's application has a bulk email system, all custom built, used by our clients to send emails based on opt-ins. While doing performance monitoring during some particularly large email batch sends, we noticed that there appears to be an artificial barrier caused by our current sending mechanism (phpMailer) For a brief time we re-routed our mail through a third-party SMTP service, but realized that they weren't sending any faster than we were. Now that the sending onus is back on us, we're thoroughly testing in anticipation of a number of upcoming big clients.
Assuming we improve the send rate for our mailer protocol (we're considering changing altogether to SwiftMailer) I was contemplating whether or not our SMTP server might also eventually become a bottleneck. What type of throughput are you able to achieve from your SMTP server? What considerations on SMTP sending (such as authentication, packaging, etc) might I have to reconsider when doing performance tweaks?