If bytes containing these instruction codes were somehow transmitted over the Internet, they could unintentionally tell the routers, switches, and other components that handled your email to take all sorts of unpredictable actions, and almost certainly cause them to crash. To protect against this problem, every email program encodes attached files before mailing them with a program that filters out any control code or instruction bytes in a predictably reversible way. When the recipient's email program receives the attachment and it is safely downloaded onto their machine, then the program decodes the attachment according to a standard procedure to reconstruct the original file. Each encoded file includes an instruction that tells the email recipient what type of encoding program was used. There are a number of more or less standard encoding methods, including MIME, uuencode, BASE64, BinHex, and AppleDouble.
Most email programs can decode most of the common standards.
Uuencode was one of the earliest encoding standards, first developed on the Unix BSD 4.0 operating system. You are sometimes given a choice in your application settings for which encoding method your email program should use. The best choice is usually MIME, which almost all email programs support. However, if somebody can't read an attachment you send them, try setting the encoding method to BinHex, Uuencode, or AppleDouble in that order. Remember to change your settings back to MIME for sending to everyone else.