Sun Java System Messaging Server 6.3 Administration Guide

5.3.2 Number of Connections per Process

The more simultaneous client connections your POP, IMAP, or HTTP service can maintain, the better it is for clients. If clients are denied service because no connections are available, they must then wait until another client disconnects.

On the other hand, each open connection consumes memory resources and makes demands on the I/O subsystem of your server machine, so there is a practical limit to the number of simultaneous sessions you can expect the server to support. (You might be able to increase that limit by increasing server memory or I/O capacity.)

IMAP, HTTP, and POP have different needs in this regard:

Note –

For more information about HTTP session security, see 23.2 About HTTP Security

Thus, at a given moment for a given user demand, Messaging Server may be able to support many more open IMAP or HTTP connections than POP connections.

The default value for IMAP is 4000; the default value for HTTP is 6000 connections per process; the default value for POP is 600. These values represent roughly equivalent demands that can be handled by a typically configured server machine. Your optimum configuration may be different; these defaults are meant only as general guidelines.

Typically, active POP connections are much more demanding on server resources and bandwidth than active IMAP connections since IMAP connections are idle most of the time while POP connections are constantly downloading messages. Having a lower number of sessions for POP is correct. Conversely, POP connections only last as long as it takes to download email, so an active POP user is only connected a small percentage of the time, while IMAP connections stay connected between successive mail checks.