This page is reached from Services → SMTP Server, then the Load Limiting tab.

Here you can set limits on how many active SMTP sessions there are, and other related settings. This lets you limit the load on the VPOP3 server. This is especially important if VPOP3 is performing spam filtering on incoming SMTP messages, because the spam filter can add considerable load. You can reduce the spamfilter load by using the QuickEnd and EarlyDNS spamfilter script settings

This setting tells VPOP3 to limit the number of incoming SMTP sessions. This does not affect internal users' SMTP connections to VPOP3, only incoming sessions.

If you set this too high, then the CPU load on the server may increase so much that sessions start timing out. This will cause the sender to re-send messages, adding even more load.

If someone tries to send an incoming message and VPOP3 is already at the limit of sessions, then VPOP3 will return an error message telling the sender to try again later.