The security firm said one in every 416 email messages received between July and September was spam compared to just one in every 3,333 emails between April and June.
Much of the increase has been caused by several large-scale malware attacks. Sophos said the worst single attack was the Agent-HNY Trojan horse, which was disguised as the Penguin Panic arcade game for Apple iPhones. However, only Windows users are affected by the viruses.