If you see this error message when you are sending messages, then it means that VPOP3 has detected too many simultaneous connections from your IP address.
This would usually happen if the sender is another mail server as they can make many simultaneous connections when there are lots of messages to send. The error code that VPOP3 gives means that the sender should try sending the messages later, so it should be possible to eventually deliver the messages unless they are arriving at this other mail server quicker than it can send them to VPOP3.
If the sender is not another mail server then it may mean that there is a bug in it where it is not closing SMTP sessions once they have been finished, thus leaving connections open until the limit is reached. This can be checked in the VPOP3 settings in Status → Active Sessions.
You can change the allowed limits on the Services → SMTP Server → Load Limiting tab. We strongly recommend not simply increasing the limits (especially 'Maximum Incoming Sessions') to very large amounts otherwise this can cause the server to become overloaded (especially if performing spam filtering) which will mean that even fewer messages can be accepted because of excessive timeouts and retries.
The 'Maximum Incoming Sessions' option detects whether sessions are incoming or not by looking at the sender's IP address. If this is allowed by the IP Access Restrictions with 'No Auth' specified, then it is treated as a local session, otherwise it is treated as an incoming session.
If lots of mail is coming from specific IP addresses (eg gateway servers), then you can reserve connections for these IP addresses. For instance, if you set 'Maximum Incoming Sessions' to 50 and 'Number of Sessions to Reserve' to 30, then 30 SMTP sessions are reserved for the specific IP addresses and the remaining 20 are available to other IP addresses.
Specify the IP addresses to reserve the sessions for in the 'Reserve Sessions for' setting - separate multiple entries with commas, and you can use wildcards, eg 192.168.1.*,127.0.0.1