Một số máy chủ SMTP mail hỗ trợ chuyển tiếp - bạn có thể nhận được thư trên một hệ thống và minh bạch để người gửi chuyển tiếp nó đến một địa chỉ hoàn toàn khác nhau trên một hệ thống khác. Ví dụ, một tin nhắn gửi đến max@ có thể được chuyển tiếp bởi các máy chủ thư để sam@. | 230 Part V Networks and Communications Some SMTP mail servers support forwarding you can receive mail on one system and transparently to the sender forward it to a completely different address on another system. For example a message sent to max@ could be relayed by the mail server to sam@. Though the idea is useful many servers implement forwarding without authenticating the sender giving spammers the opening they need. We talk about spam and what you can do about it later in this chapter. The worst thing about raw SMTP is that it accepts only text messages not binary files. People commonly want to mail arbitrary files however and send text that includes fonts colors and other formatting. Three approaches to handling this requirement are common UUE MIME and HTML User-user encoding UUE It s possible to recast the binary data stream you want to send differently. For example you could take every 6 bits creating numbers in the range from 0 to 63 and remap the resulting numbers onto the printable characters. This expands the data stream producing 8 bits from every 6 but it results in a new data stream that contains nothing but text characters acceptable to SMTP. This was the original way of sending binary data through SMTP on the Internet encode the data mail the text and decode at the other end. Current-generation electronic mail client programs such as Windows Messaging which is included with Windows support this transformation automatically. Multipurpose Internet Mail Extensions MIME Internet software like Web browsers actively know what sort of data is stored in different kinds of files that EXE files are executables ZIP files are compressed archives WAV files are sound clips and so on. The MIME coding standard for electronic mail allows the properties of files to be sent along with the files themselves. Technically MIME uses the same approach UUE does expanding a smaller number of bits to a larger number that transforms strictly to .
