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This article mainly talks about "what character replacements are there in the smtp protocol". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn what characters are replaced in the smtp protocol.

Internet email is not a perfect system. The message may be damaged in several stages of delivery to the final destination. Specifically, e-mail sent over the Internet may span many network technologies. Many network and mail technologies do not support all possible functional environments in SMTP transport. Messages passing through these systems are likely to be modified so that they can be transported.

There are many widely deployed MTA that do not meet the requirements on the Internet. These MTA use the SMTP protocol and can implement change messages at any time using the internal data structures of their hosts, or simply break them.

The following guidelines may be useful to anyone who changes the data format (media type), which should be able to withstand the broadest range of network technologies and known corrupted MTA. Note that content encoded in any base64 manner will meet these rules, but some well-known mechanisms, especially UNIX uuencode tools, will not. Also note that anything encoded in Quoted-Printable is lossless on most gateways, but some gateways may not connect to systems that use the EBCDIC character set.

(1) in some cases, the encoding used for data may be changed as part of a normal gateway or user agent operation. In particular, the conversion from base64 to Qp encoding and vice versa may be necessary. This may cause the CRLF sequence to be confused with the line and break in the body of the text. Therefore, CRLF can never be defined as something other than as the Terminator of a line.

(2) many systems can choose to describe and store text data using local new conventions. The local new convention may not conform to RFC822's CRLF convention-known systems use normal CR, normal LF, CRLF, or count records. As a result, individual CR and LF characters are not universal; they may be lost or converted to delimiters on some systems, and therefore cannot be used.

(3) the transmission of NULs (US-ASCII value 0) is a problem in Internet mail. This is largely the terminating character of NUL as a standard runtime library commonly used in many C languages. The habit of using NUL as termination characters is now deeply ingrained, and email messages should not rely on them being saved.

(4) TAB (HT) characters may be misunderstood or may be mistakenly automatically converted to a variable number of spaces. This is inevitable in some environments, especially those that are not based on the US-ASCII character set. This conversion is highly disapproved, but it can happen, so the message format cannot rely on TAB (HT) characters for long.

(5) lines longer than 76 characters may be wrapped or truncated in some environments. Line feeds or forced truncation of lines during mail delivery are highly disapproved, but in some cases they are inevitable. Applications that require long rows must somehow distinguish between soft and hard breakpoints of line data. A simple way is to use quoted-printable coding. )

(6) using the "blank space" character (TAB (HT)) on a row of data may be discarded by the transport agent, while other transport agents may use these characters to populate these lines of data so that all lines in the mail file are equal in length. Therefore, the persistence of the following white space must not be relied on.

(7) many mail domains use variants of the US-ASCII character set, or use the EBCDIC character set, such as the EBCDIC character set, which contains most but not all US-ASCII characters. The character translation gateway cannot rely on the correct translation of characters that are not in the "unchanged" set. For example, there is a problem sending undecoded messages to BITNET (the World Education Network "Bitnet"), which is an EBCDIC system. Similar problems can still occur without traversing gateways because many Internet hosts use character sets other than US-ASCII. The definition of printable strings adds some special restrictions in X.400. Only characters are consistent in all known gateways, the characters corresponding to uppercase and lowercase, the letters Amurz and az-,10 digits 0-9, and the following eleven special characters:

"'" (US-ASCII decimal value 39)

"(" (US-ASCII decimal value 40)

")" (US-ASCII decimal value 41)

"+" (US-ASCII decimal value 43)

"," (US-ASCII decimal value 44)

"-" (US-ASCII decimal value 45)

"" (US-ASCII decimal value 46)

"/" (US-ASCII decimal value 47)

":" (US-ASCII decimal value 58)

"=" (US-ASCII decimal value 61)

"?" (US-ASCII decimal value 63)

The simplest message will limit itself to relatively short lines of text, which are made up of the 73 character sets described above. Base64 coding follows this rule.

(8) some mail transfer agents destroy the data of strings containing certain letters. In particular, a row of data is known to be corrupted by some SMTP servers, and a line starting with five characters "From" (the fifth character is a space) is often corrupted. A rigorous agency organization can prevent data corruption caused by encoding data (for example, using "= 46rom" in QP encoding instead of "From" and "= 2e" in a row starting with "From" instead of a separate period (".") on a line). Note that the above list is not the practice of the list recommended by MTAs. MTA forbids changing white space or truncating a long row of data in RFC 821. These bad habits and practices exist on established networks, but the implementation should be robust when dealing with the adverse effects they may cause.

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