4.2 I have recently installed Postfix at my ISP and when I start it up it will only run for an hour or so. I think we have too many users on it, (or something) because it spirals steadily down, ending up with no swap and no memory left. Is there a way that we could set a limit on the number of connections, or processes that Postfix can start?

Wow! That's unusual. Because Postfix is so fast and uses so little memory this is rarely seen. You may have an under-powered machine for the job you've given it. But until you can upgrade the box you can set the default_process_limit parameter (the default is 50) which gives direct control over inbound and outbound delivery rates. This parameter controls the number of concurrent processes that implement a Postfix service (SMTP client, SMTP server, local delivery, etc.). On small systems, or on systems connected via dialup networks, a default_process_limit of 10 is probably more than enough. Use a larger value if your machine is a mail hub. You can override this setting for specific Postfix daemons by editing the master.cf file.

If you're providing POP service from the same box you also may want to take a look at the memory usage of your POP3 daemon and consider changing it to a version that can run in stand-alone mode. Gnu-POP3d, by Jakob "sparky" Kaivo, jkaivo@nodomainname.net seems to do a nice job and it has a very small footprint. Here's the URL: NoDomainName Networks.

As an alternative to this you might want to consider splitting the POP3 service away from this machine which would definitely help if you're in a high volume environment.