Complex sentence: After most commercial mail servers began checking mail with anti-virus software and filtering certain malicious file types, EXE, RAR, Zip, 7-Zip, mail server software was then configured to unpack archives and check their contents as well.
Split into simpler sentences: After most commercial mail servers began checking mail with anti-virus software and filtering certain malicious file types, EXE, RAR, Zip, 7-Zip. Mail server software was then configured to unpack archives and check their contents as well.