When conducting electronic correspondence, mail servers strive to filter out unsolicited mail (spam) as efficiently as possible. From time to time it happens that these spam filtering services get blocked by letters that do not contain anything that would justify this. Therefore, it may happen with us that we cannot send an email to a recipient, or that we cannot receive mail from others on our own server. In this description, we will investigate one of the more common server-side "554 5.7.1 Service unavailable; Client host blocked using zen.spamhaus.org; Error: open resolver;" type error.

After updating ISPConfig, not all services restarted. In this case, the Dovecot POP3/IMAP mail server and even Postfix threw warnings. Although we fixed these errors earlier, our settings were overwritten again during the ISPConfig update. In this troubleshooter, on the one hand, we fix "Address family not supported by protocol" type errors that cause the service to be inoperable, and we also make their permanent settings so that they remain even after further ISPConfig updates.

.htaccess files are very useful accessories for the Apache web server, with the effective use of which we can solve many problems. However, in the case of a more complex server configuration, we need to pay attention to several things, otherwise we can easily generate Apache errors.
In this troubleshooter, we deal with redirects, where in certain situations we can end up in an endless redirect cycle, which eventually results in "AH00124: Request exceeded the limit of 10 internal redirects due to probable configuration error." leads to content errors.

Linux services are programs running in the background, which do not have a separate user interface, but respond to the requests of other programs through some communication mechanism (mostly on the network). Services in modern Linux systems are managed by systemd, and they can be started, stopped, masked, unlocked, etc. using the systemctl command. There are cases when the systemctl command cannot unmask, in which case a manual solution must be applied.

In this troubleshooter, we roll up a Fail2Ban block caused by an Apache error of type "AH01630: client denied by server configuration" caused by Matomo's web analytics system. Finally, we fix the problem by modifying the Fail2Ban filter in apache-auth.

VirtualBox can be installed on a variety of operating systems, including all versions of Windows. However, there are still issues with Windows 11. The current version of Virtualbox (6.1) theoretically supports Windows 11, but there are problems during installation. This troubleshooter resolves this issue so that you can install the current latest version of Windows, 11, without any problems.

If there is a major change to your Debian system - so you don't just need to upgrade one or two sub-versions - you will get an error message during a smooth upgrade:
This must be accepted explicitly before updates for this repository can be applied. See apt-secure (8) manpage for details.
This troubleshooter will fix this.

When you are not using the latest version of a Linux distribution, oddities may arise. For example, in Debian 9, you may receive a "public key not available" (NO_PUBKEY) error message when updating the backports package. We will remedy this problem in this short troubleshooter.

If you frequently use virtual machines with VirtualBox, it is a more convenient and time-saving solution if you do not create and configure a new virtual machine for each installation, but use the existing ones. In some cases, this can cause problems, such as when you try to start your installer in EFI mode, but it does not want to start. In this short little troubleshooter, we will look at how to start ISO installers on EFI mode virtual machines.

When you connect an external USB drive or flash drive to your machine, the Linux system will automatically mount its file system for the appropriate user. However, this automatic mount may fail for some reason and you will get a small window with an "Not authorized to perform operation" error message.