differences between sendmail and postfix

explain the differences between sendmail and postfix.

*Sendmail
Sendmail used the milter protocol to communicate with the PureMessage filter (also called a milter). This means that each sendmail child requires a dedicated milter process to make the decision to pass/quarantine or reject a message. It is a continuosly transaction with the milter and easily run out of RAM when reached max daeman. By default, sendmail receives incoming SMTP connections on port 25 and communicates with PureMessage using the milter protocol on TCP port 3366.
Inbounce/Outbounce Message <--> Internet <--> Sendmail (RAM dependence) <--> Milter
Advantages
real time less IO on the hardrives. fast message delivery
Disadvantages
Puts a Cap on the number of concurrent connections you can handle. CPU intensive Memory intensive does not absorb message spikes and relies on temp fails and server throttling to control them.

*Postfix
Postfix on the other hand communicates with the filter using the SMTP protocol. The way this behaves it that postfix will receive a messages, queue it, and then send it to the filter. this has a bunch of implications. As you have already mentioned on your email, Postfix uses 2 ports. By default, Postfix receives incoming SMTP connections on port 25 and communicates with PureMessage using the content_filter mechanism on ports 10025 and 10026.
Inbounce Message --> Internet --> Holding Area (Disk dependence) --> MilterInternet <-- Holding Area (reply port) <-- Milter <-- Outbounce Message
Advantages
postfix is really good with message spikes, it will absorb spikes and pass them internally to a pool of milters. Lower CPU utilization Lower RAM utilization This in turn allows for less hardware to handle more traffic. Postfix will handle frond side queuing for the milter and will not tax the system by spawning more and more filters.
Disadvantages
postfix will queue first, this means more IO overhead