For larger installations, the number of users that can be served simultaneously is an issue of a very high concern. In order to estimate how many users you can serve at the same time, you should realize what type of service your clients will use.
POP3 mailers connect to the server just to download new messages. Based on the average connections speeds, expected mail traffic, and your user habits, you can estimate how much time an average session would take. For example, if you are an ISP and you estimate that an average your "check mail" operation will take 15 seconds, and they mostly check their accounts an average of 2 times each during 12 peak hours, then with 100,000 POP3 users you can expect to see 100,000 * 2 * 15 sec / (126060 sec) = ~70 concurrent POP3 sessions.
This number is not high, but POP3 sessions put a high load on your disk I/O and network I/O subsystems: after authentication, a POP3 session is, essentially, a "file downloading" type of activity.
The IMAP protocol allows a much more sophisticated processing than POP3. Mail is usually left on the server, and some unwanted messages can be deleted by users without downloading them first.
The IMAP protocol is "mail access", not "mail downloading" protocol. IMAP users spend much more time being connected to the server. In corporate environments, users can leave their IMAP sessions open for hours, if not days. While such inactive sessions do not put any load on your disk or network I/O subsystems or CPU, each session still requires an open network connection and a processing thread in the server. Since the IMAP protocol allows users to request search operations on the server, IMAP users can also consume a lot of CPU resources if they use this feature a lot.
When the server needs to handle many IMAP or POP connections, it is important to configure more IMAP and POP channels, to allow for large numbers of users to connect concurrently. Some modern IMAP clients and the MAPI connector may even open multiple connections for a single account, and each is counted in the IMAP channel total. Fortunately, IMAP and POP channels are created only when used, so no resources are consumed if the IMAP and POP channels are set to 10000 if only 2000 are being used - however, be careful to set this value below the threshold where your system will be unable to withstand further connections, and could become unresponsive for users already connected. The IMAP and POP channels setting provides a limit for protecting your system or cluster resources from being overwhelmed, in the case of a peak load or denial of service (DoS) attack.
The CommuniGate Pro WebUser Interface provides the same features provided by IMAP mailer clients, but it does not require an open network connection (and processing thread) for each user session. When a client (a browser) sends a request, a network connection is established, the request is processed with a server thread, and the connection is closed.
This allows the Server to use just 100 HTTP connections to serve 3,000 or more open sessions.
CommuniGate Pro also supports the HTTP 1.1 "Keep-Alive" option, located on the WebUser Interface Settings page as "Support Keep-Alive". HTTP Keep-Alive sessions for WebUsers will cause each WebUser session to maintain one or more open connections from the user client to the server for the entire session duration. This method is not recommended on a busy, high-volume server as it will consume significant CP