EDIT: [RESOLVED] Server did not have enough memory to handle the antivirus system - see the thread for details. TL;DR: DO NOT operate ISPConfig 3 on server with less than 2 GB of RAM and 2 GB swap. Hello Team, I have installed my server some years ago using this howto: https://www.howtoforge.com/tutorial...ix-dovecot-mysql-phpmyadmin-rkhunter-binutils The server itself is actually Debian 8.11 I think it's it's irrelevant but let me say that for emails I use Thunderbird with SOCKS5 proxy set to 127.0.0.1 - the proxy itself is fine as emails sent via servers other than server1.mydomain.com work fine. I don't use TOR or any other frowned upon network as SOCKS5 proxy. The issue: My server suddenly stops receiving emails - at this point I can't be sure if it's really 100% of them but it was at least 90%. From the 3rd party the sent email looks like this (this is what they received after couple of days since they sent the email): Subject: Undelivered Mail Returned to Sender Body: Temporary fix: Reboot of the server. If I reboot the server the emails that could not be delivered and are in queue at the 3rd party servers (I assume) and are eventually coming through over the next couple of hours. What I checked: Disk space and host utilization. Server has gigabytes of SSD space available and there is enough RAM as well as space in swap available, CPU utilization is low. What I also noticed: It seems that the issue goes both ways - at least in one case a sent email from server1.mydomain.com has been also bounced by Gmail, however this might be caused by reputation downgraded by some blacklists as my emails were bouncing (checked with mxtoolbox.com - the server IP already made it to the blacklist for reason of too many bounced emails): Subject: Undelivered Mail Returned to Sender Body: This issue has already happened maybe 3 months ago but I did the reboot and forgot about the issue now the issue is back I really don't know what to check or how to even diagnose this. Thank you very much in advance for any help.