In the mid 1980s text files could be grouped with UNIX tools such as bundle[1][2] and shar (shell archive)[3] and included in email message bodies, allowing them to be unpacked on remote UNIX systems with a single shell command.
This is because of a number of potential limits: The result is that while large attachments may succeed internally within a company or organization, they may not when sending across the Internet.
A lot of malware is distributed via email attachments with some even considering such to be the main vector for cyberattacks on businesses.
[14][17][18] While many email servers scan attachments for malware and block dangerous filetypes, this should not be relied upon − especially as such cannot detect zero-day exploits.
However, in practice this advice is not enough – "known trusted sources" were the senders of executable programs creating mischief and mayhem as early as 1987 with the mainframe-based Christmas Tree EXEC.
Since the ILOVEYOU and Anna Kournikova worms of 2000 and 2001, email systems have increasingly added layers of protection to prevent potential malware.