I'm testing this on a new VPS with only the mail service turned on. All mailbox users use Outlook, when only 10 users use it at the same time, there is no abnormality, when there are 20 users use it at the same time, Outlook will prompt to receive an error, for example, it can't connect to the server, when the abnormality occurs, all the users are abnormality at the same time. When I restarted the VPS, it went back to normal, but the problem resurfaced a few minutes later. So, is there a concurrency limit? Or is there some other reason? Thanks!
There are no such low connection limits, but maybe your VPS runs out of resources or its related to your VPS hoster. If its related to your server and not your network or the way you connect Outlook, then you should find a hint on what is causing this in the mail.log or syslog file of your server.
Due to time constraints, I rebuilt the VPS, ubuntu 22.04, and reinstalled ISPCONFIG. But a few hours later, the problem reappeared, lasted about 10 minutes or so, and then returned to normal. Seven hours later, the problem reappeared, again for about 10 minutes, and returned to normal. I checked to see if the VPS was running out of resources and it seemed to be fine, and I also checked with the service provider and the answer was also fine. So I'm still analyzing the mail.log and syslog you mentioned, or anything else that might be relevant, and so far I haven't figured out why. I will analyze it again the next time the same problem occurs. Because so far everything is fine, the last time I had a problem was 22 hours ago. Tip. Webmail is working fine when this problem occurs, no problem. Thank you for your help.
