First, about hardware requirements.
What you're proposing is absolute overkill for such a small client load. You won't need to upgrade the hardware :-)
About resource limits of _cyrus user and sysctl values, are there well known values? Should I increase kern.maxfiles for example? I wouldn't like to learn it at production time.
Again, given the minimal load from IMAP, the out of the box defaults will do just fine.
Well, this are my questions. May be the hardware is overkill for our load, but sizing hardware without prior experience it's always a difficult task, so if anybody wants to share their experience...
Cyrus has a very small CPU and memory footprint. All you need to ensure is that you have enough I/O bandwidth from the disk, through the imapd process, and out the network interface. From what you're describing, you have nothing to worry about.
Sendmail can want memory when delivering messages with large numbers of recipients (e.g. mailing list expansion), but again, it's doubtful your load will even begin to stress the hardware.
--lyndon
