Spam email is defined as unsolicited messages sent indiscriminately in bulk to many recipients. It is commonly known in Japanese as "meiwaku mail" (literally "nuisance mail"). Most spam emails contain unsolicited advertisements, typically promoting dating services, adult websites, pyramid schemes, or multi-level marketing campaigns.
However, spam emails often pose additional threats by attaching malware-infected files. A recent trend (notably from around 2015) highlights an increased use of fake Microsoft Office files or PDFs containing macros that, when executed, download malware onto devices. Additionally, spam disguised as invoices or postal tracking notifications has become common, often deploying ransomware that infiltrates systems and demands ransom payments. Emails containing malicious URLs leading to phishing websites have also notably increased, trapping unsuspecting victims in fraudulent schemes.
A key method of defense against spam emails involves utilizing email filtering technology. Filtering typically identifies spam by detecting patterns or keywords common to unsolicited emails, after which it modifies the email subject line to indicate spam or isolates these emails in dedicated spam folders. Bayesian filters are widely employed for email classification, and in recent years, the accuracy of such filtering software and services has significantly improved.
In addition, DNS-based anti-spam measures, such as DNS Blacklists (DNSBL), are increasingly used. These lists publicly document IP addresses associated with spam activities. Email servers querying DNSBL services can determine whether the sender's IP address is listed, allowing administrators to block or reject mails accordingly.