According to a recent report by Symantec, Linux systems are 5 times more likely to send spam than Windows. The finding which were published in the Symantec MassageLabs April Intelligence Report were based on Symantecs identifying the operating system of a machine sending spam, then calculating the ratio of spam from each operating system and compared it with each system’s market share.
92+% of spam originated from Microsoft Windows systems, however they have a 91+% share of the operating system market share. Whereas Linux systems originate 5+% of spam and they have 1.03% of the operating system market share. Relative to their market share Symantec’s calculations indicated that Linux have a ‘spam index’ of 4.99 where Windows have a ‘spam index’ of 1.01
These figures do not necessarily indicate that Linux us being targeted by spammers or is considerably less secure than Windows. However Linux is an operating system often used to run email relay systems. Symantec suggest that the problem could be that the relays have been established without adhering to basic anti-spam precautions. In most cases the spam came from a machine running open source mail transfer agents such as Postfix and were not restricting access to port 25.