电子邮件标签与本地部分和连字符

Question

我一直在阅读 RFC、摘要、维基百科等。我对本地部分与标签非常困惑。在我看来，本地部分在@之前。这似乎很简单。标签是由点分隔的域的任何部分。但是在我看来，有些地方也将本地部分称为标签。在允许使用连字符的情况下，这非常令人困惑。那么具体什么是标签呢？

那么，其中哪些是有效的电子邮件地址（如果有）？

-bobross@painting.com
bobross-@painting.com
bobross@-painting.com
bobross@painting-.com

我的理解是标签既不能以连字符结尾也不能以连字符开头，并且不能包含两个连续的连字符。我错过了什么吗？

奖励积分 - 本地部分允许使用许多特殊字符，但我看到的一些消息来源说本地部分必须以字母数字字符结尾，但我实际上没有看到在任何标准中..我是否遗漏了它或者它可以以允许的字符之一结尾？

Answer 1

来自 rfc5321, section 2.3.5:

A domain name (or often just a "domain") consists of one or more
components, separated by dots if more than one appears.  In the case
of a top-level domain used by itself in an email address, a single
string is used without any dots.  This makes the requirement,
described in more detail below, that only fully-qualified domain
names appear in SMTP transactions on the public Internet,
particularly important where top-level domains are involved.  These
components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
for SMTP purposes to consist of a sequence of letters, digits, and
hyphens drawn from the ASCII character set [6].  Domain names are
used as names of hosts and of other entities in the domain name
hierarchy.  For example, a domain may refer to an alias (label of a
CNAME RR) or the label of Mail eXchanger records to be used to
deliver mail instead of representing a host name.  See RFC 1035 [2]
and Section 5 of this specification.

换句话说，abc.def.xyz 的域由 3 个组件（又名标签）组成：abc、def 和 xyz。每个标签只允许包含字母、数字和连字符。对于更具体的定义，我们必须检查 Command Argument Syntax 部分中的 ABNF 语法，因为我们真正关心的是 MAIL FROM 和 RCPT TO 命令的参数语法（又名“外行术语中的电子邮件地址”令牌）。

“电子邮件地址”在 SMTP 规范中实际上称为 Mailbox：

Mailbox        = Local-part "@" ( Domain / address-literal )

现在看一下 Local-part 令牌的定义：

Local-part     = Dot-string / Quoted-string
               ; MAY be case-sensitive


Dot-string     = Atom *("."  Atom)

Atom           = 1*atext

要获得 atext 的定义，我们需要查看 Internet Message Format. Specifically, we need to look at section 3.2.3:

atext           =   ALPHA / DIGIT /    ; Printable US-ASCII
                    "!" / "#" /        ;  characters not including
                    "$" / "%" /        ;  specials.  Used for atoms.
                    "&" / "'" /
                    "*" / "+" /
                    "-" / "/" /
                    "=" / "?" /
                    "^" / "_" /
                    "`" / "{" /
                    "|" / "}" /
                    "~"

（注意：我省略了 Quoted-string 的定义，因为它与您的问题无关。）

现在让我们检查一下 Domain:

的定义

Domain         = sub-domain *("." sub-domain)

sub-domain     = Let-dig [Ldh-str]

Let-dig        = ALPHA / DIGIT

Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig

我们在这里看到的是 label 与 sub-domain 标记相同，但在这里我们还看到 labels[= Domain 的 76=]（又名 sub-domain's）不能以连字符开头或结尾。

我们还可以看到 sub-domain 标记实际上是 Atom 中允许字符的子集，因此每个组件中允许的字符Local-part 与 Domain.
的每个组件中允许的不同
回答您的其他问题：

And with that, which of these are valid email address (if any)?

-bobross@painting.com [VALID] bobross-@painting.com [VALID] bobross@-painting.com [INVALID] bobross@painting-.com [INVALID]

My understanding is that a label can neither end or begin with a hyphen and it must not contain two consecutive hyphens. Am I missing anything with that?

让我们使用术语 sub-domain 来减少混淆。

是的，没错。

Bonus points - there are a number of special characters allowed in the local-part, but some sources I've seen say that the local-part must end in an alphanumeric character, but I'm not actually seeing that in any standard..am I missing it or can it end with one of the allowed characters?

嗯，根据rfc5322的ABNF语法，一个Local-part可以同时以和以连字符结尾。

电子邮件标签与本地部分和连字符

Email label vs local-part and hyphens

email

rfc5322