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📢 | Kawasaki City: Current status, issues and future development of Kawasaki Port Development Project [JPI Seminar held on October 10 (Wednesday)]…


Kawasaki City: Current status, issues and future development of Kawasaki Port Development Project [JPI Seminar held on October 10 (Wednesday)]…

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Japan Planning Institute will hold the following seminar by inviting Mr. Kazuo Nakagami, Director of Port Bureau, Kawasaki City.Semi ... → Continue reading


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Wikipedia related words

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Email Address

Email Address(English: electronic mail address),Email address(Email address,English: email address, e-mail address) Is電子 メ ー ルInSENDIndicates the destination or source. As an abbreviationア ド レ ス,E-mail address,Emailand so on.


The email address has the following syntax:

Local department@Domain (eg:foo@example.com

The structure of the local part isMail serverAccording to the specifications of. Basically it is a user name or an alias of the user name. Domain ishostname(Assigned to hostDomain name) And specify the mail server. In this example, ""example.comRegistered in the mail server that can be specified by "foo"User" is the basis. As a document that defines the format of the email address,RFCRFC 5321 (Simple Mail Transfer Protocol) [1]And RFC 5322 (Internet Message Format) [2]Exists and defines the characters that can be used in email addresses[3].

Characters that can be used in the local part

The characters that can be used in the local part areASCIIIt is a character.

  1. First, nextASCIIA format in which characters are arranged as is (in RFC 5321,Dot-string, In RFC 5322dot-atomCan be used).
    • Big and smallLatin letters(initiallyuppercase letter-LowercaseAre distinct, but in practice most implementations do not[4].. (Typically written in lower case)
    • 数字
    • ! # $ % & ' * + - / = ? ^ _ ` { | } ~(actually,ProviderIn many cases, the symbol characters that can be used on the side are limited to only a part)
    • .(It is possible to use it at other than the beginning and the end. It cannot be two or more in a row.
  2. further,"" "quoted-stringThe following ASCII characters can be used in addition to[5].
    • ( ) < > [ ] : ; @ ,
    • .(There is no limit in quoted-string)
    • Space
  3. Furthermore, in quoted-string, “\In front ofquoted-pairThe following ASCII characters can be used in addition to this format.
    • \ "

For example,RFC[which one?]This is a valid email address under the provisions of.

Abc@example.com (dot-atom) Abc.123@example.com (dot-atom) user + mailbox / department = shipping@example.com (dot-atom)! # $% &'* +-/ =? ^ _ `. {|} ~@Example.com (dot-atom)[6]
"Abc@def"@example.com (quoted-string) "Fred \ Bloggs" @ example.com (quoted-string including quoted-pair) "Joe. \\ Blow" @ example.com (quoted-pair Included quoted-string)

RFC5322 p.12 definition

 dot-atom-text = 1*atext *("." 1*atext)

So the following is an invalid email address, as the character is always required after the "."[7].

Abc.@example.com (“.” is used at the end of the local part) Abc..123@example.com (“.” is continuous)

Maximum length of local part is 64 characters[8].. (Actually, the settings on the provider side,Mail software(In many cases, the maximum length of the local part is limited to 20 to 30 characters due to restrictions such as

Characters that can be used in domains

The domain can be in one of the following formats:

The maximum domain length is 253 characters, and the maximum email address length is 254 characters.[8].

# Characters that can be used in the local partAs mentioned above, if the local part is not in quoted-string format, “."Cannot be used at the beginning and end or cannot be used more than once in succession. However, some implementations (example:Mobile phoneE-mail) deviates from this specification, and there are cases where special characters outside the specifications can be used.

PostfixIf an email address that has a local part that deviates from the specifications exists in the envelope or header of the email to be delivered," "It will be transformed into a bracketed form. You can disable this function for the email address in the envelope.[11]However, the email address in the header cannot be invalidated.

Use email address in email

Email addresses are used in several places in email and appear in a format that depends on where they are used.

Where email addresses are used in email

  • envelopeInside the path. Defined in RFC 5321.
    The sender of the email. Also called the envelope from.
    The destination of the email. Also called the envelope to.
  • Mail headerInside field. Defined in RFC 5322.
    From field
    Email author.
    Sender field
    The sender of an email when the sender of the email is different from the author or there are multiple authors.
    To field
    The recipient of the email.
    Reply-To field
    Email reply to.
    carbon copyRecipients of.
    Bcc field
    Blind carbon copy recipient.
    Resend field (Resent-From/Resent-Sender/Resent-To/Resent-Cc/Resent-Bcc)
    Used to fill in information when re-sending an email without changing the original fields. Each corresponds to a field without "Resent-".
    Return-Path field
    The reverse-path information is filled in by the SMTP server.

Format used in email

The following table shows the formats of email addresses that can be used at each location.

List of email address usage formats
形式RFC 5321 designationWhere to use (envelope)RFC 5322 designationLocation (header)[1]
foo@example.comMailbox-addressmailboxaddr-specFrom Sender Reply-To To Cc Bcc
foo <foo@example.com>
 foo <foo@example.com>,
 bar <bar@example.com>;
--groupReply-To To Cc Bcc
<foo@example.com>Pathreverse-path forward-pathpathReturn-Path
^The format of the e-mail address that can be used in the resent fields (Resent-From, Resent-Sender, Resent-To, Resent-Cc, Resent-Bcc) is the same as the corresponding field without "Resent-".
  • "Local department@domainFormat (eg: foo@example.com) In RFC 5321Mailbox (Mailbox), and in RFC 5322addr-specCall.
  • For header fields other than Return-Path among the header fields that use email addresses, addr-spec and addr-spec format are< >”, and insert a display name beforename-addr(Example: <foo@example.com>,foo <foo@example.com>) Can be used. In RFC 5322, addr-spec and name-addr are combined.Mailbox (mailbox).
  • In the From field, enter the mailbox,Multiple authors can be entered using a "-separated list of mailboxes, where the Sender field with one sender's mailbox is required.
  • In the Reply-To, To, Cc, Bcc fields, enter the mailboxes of multiple people.,Separate with ", display name and ":",behind";"I insertedグループ (group) (Example: foobar:foo <foo@example.com>,bar <bar@example.com>;) Can also be used, and RFC 5322 combines mailboxes and groups.ア ド レ ス (address). In the Reply-To, To, Cc, Bcc fields, enter the address,Multiple entries can be entered using a list of addresses separated by "(mailboxes are also addresses, so a list of mailboxes can also be used). The value of the Bcc field can be empty.
  • For the reverse-path and forward-path, use the Mailbox format “< >path (Path) (Example: <foo@example.com>) Is used. The Return-Path field is similar.

Special email address

Used as the email address of the administrator of the email system. The SMTP server uses "Postmaster@"Domain" or "no domain"PostmasterYou must receive an email with "specified. Both"PostmasterThe "" character is not case sensitive.
Null address
Mail transfer agent (MTA) informs the undeliverable e-mail to the reverse-path of the e-mail when the e-mail cannot be delivered due to the wrong address etc.Bounce emailTo ship. The bounce email reverse-path has a null value (zero) Path using the address of<>Is used.
sendmail, Postfix,qmailMTA, such as in the From field of the bounce mail, "MAILER-DAEMONUse the email address that you used.

others,RFC 2142 (Mailbox Names for Common Services, Roles and Functions) [12]The frequently used email addresses are defined in.


[How to use footnotes]
  1. ^ Klensin, J., ed. (Oct. 2008), Simple Mail Transfer Protocol, RFC 5321 
  2. ^ Resnick, P., ed. (Oct. 2008), Internet Message Format, RFC 5322 
  3. ^ RFC 5322 Considered the test data of the email address (local-part) along & 5321 https://qiita.com/yoshitake_1201/items/40268332cd23f67c504c
  4. ^ RFC 5321 Section 2.4 “General Syntax Principles and Transaction Model” and Section 4.1.2 “Command Argument Syntax” define mailboxes that require quoted-strings and mailboxes that have a case-sensitive local part. Doing so is interfering with interoperability and should be avoided.
  5. ^ RFC 5321 According to Section 4.1.2 “Command Argument Syntax”, defining mailboxes with local parts that require quoted-string format should be avoided as it interferes with interoperability.
  6. ^ However, rfc976 https://tools.ietf.org/html/rfc976 In UUCP Mail Interchange Format Standard, there is a bang path (connect the host name with! To represent the email address). An example of a bang path is hosta! Hostb! User.
  7. ^ RFC 5322 Internet Message Format, IETF, 2008 https://www.rfc-editor.org/rfc/rfc5322.txt
  8. ^ a b RFC 5321 Section “Size limits and minimums”
  9. ^ RFC 5321 Section 3.6 “Relaying and Mail Routing”
  10. ^ RFC 5321 exemplifies with a real IP address, but hereIP address for exampleI used.
  11. ^ "smtp_quote_rfc821_envelope“Parameter”noSet to "("Postfix configuration parameters”(Japanese). Postfix page (September 2007, 6). April 2007, 12Browse.reference).
  12. ^ Crocker, D. (May 1997), Mailbox Names for Common Services, Roles and Functions, RFC 2142 

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