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Recently, I had to deal with a serious problem: backscatter. One of our mail gateways ended up listed on the backscatter.org blacklist for sending bounce messages to forged senders. After checking the logs, I quickly realized that our system wasn’t actually protected against backscatter attacks, so I had to do something about it. What Even Is Backscatter? Backscatter is unwanted email that your mail server sends after receiving a message, usually in the form of a non-delivery report (NDR) or bounce to a forged sender address. ...

Understanding dovecot.index.cache I recently migrated an old mail server system into its new home. After the migration, I checked the logs and noticed some warnings that looked like this: May 17 11:23:13 server1 dovecot: dsync-local(user@domain.tld)<cRjZCwGnKWiIvicA2dm5Tw>: Error: Mailbox INBOX: mmap(size=511310568) failed with file /var/vmail/domain.tld/user/Maildir/dovecot.index.cache: Cannot allocate memory The error indicates that the dovecot.index.cache file is too big to process, and Dovecot cannot allocate enough memory to handle it. What Are dovecot.index.cache Files? Dovecot, the most popular IMAP server, uses a set of index files (dovecot.index, dovecot.index.cache, dovecot.index.log, etc.) to speed up mailbox access. The file I had to deal with stores cached message metadata (headers, flags, and preview text) so Dovecot does not have to read each message file in the mailbox every time. ...

Welcome to My Blog! I’m Demian, a Sysadmin, Email Infrastructure enthusiast, and a passionate Open Source contributor. This is where I’ll be sharing what I know about system administration, managing email infrastructure, and contributing to open source projects. What to Expect On this blog, I’ll be writing about: Sysadmin tips and tools: Everything I’ve learned managing servers, networks, and infrastructure. Email Infrastructure: Best practices for setting up, securing, and managing email systems. Open Source: How I contribute to open source projects, and tips for getting started if you want to do the same. This blog is a space to share knowledge, troubleshoot common issues, and explore new tools and techniques. Whether you’re just getting into system administration, looking for email setup guides, or interested in contributing to open source, you’ll find something useful here. ...