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　ユーザー登録しようとしても、メールアドレスに"+"が入っていると、受け付けてくれない登録フォームの多いこと。
　"+"の使用はRFC的に問題ないはず。
　むしろ"+"を受け付けない方がRFC違反。
　以下は、RFC2822の抜粋。引用の関係で、順番が逆になっていますが。

RFC2822
3.4.1. Addr-spec specification

An addr-spec is a specific Internet identifier that contains a
locally interpreted string followed by the at-sign character ("@",
ASCII value 64) followed by an Internet domain. The locally
interpreted string is either a quoted-string or a dot-atom. If the
string can be represented as a dot-atom (that is, it contains no
characters other than atext characters or "." surrounded by atext

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RFC 2822 Internet Message Format April 2001

characters), then the dot-atom form SHOULD be used and the
quoted-string form SHOULD NOT be used. Comments and folding white
space SHOULD NOT be used around the "@" in the addr-spec.

addr-spec = local-part "@" domain

local-part = dot-atom / quoted-string / obs-local-part

domain = dot-atom / domain-literal / obs-domain

domain-literal = [CFWS] "[" *([FWS] dcontent) [FWS] "]" [CFWS]

dcontent = dtext / quoted-pair

dtext = NO-WS-CTL / ; Non white space controls

%d33-90 / ; The rest of the US-ASCII
%d94-126 ; characters not including "[",
; "]", or "\"

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RFC 2822 Internet Message Format April 2001

backslash characters may appear in a comment so long as they appear
as a quoted-pair. Semantically, the enclosing parentheses are not
part of the comment; the comment is what is contained between the two
parentheses. As stated earlier, the "\" in any quoted-pair and the
CRLF in any FWS that appears within the comment are semantically
"invisible" and therefore not part of the comment either.

Runs of FWS, comment or CFWS that occur between lexical tokens in a
structured field header are semantically interpreted as a single
space character.

---

3.2.4. Atom

Several productions in structured header field bodies are simply
strings of certain basic characters. Such productions are called
atoms.

Some of the structured header field bodies also allow the period
character (".", ASCII value 46) within runs of atext. An additional
"dot-atom" token is defined for those purposes.

atext = ALPHA / DIGIT / ; Any character except controls,
"!" / "#" / ; SP, and specials.
"$" / "%" / ; Used for atoms
"&" / "'" /
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~"

atom = [CFWS] 1*atext [CFWS]

dot-atom = [CFWS] dot-atom-text [CFWS]

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

Both atom and dot-atom are interpreted as a single unit, comprised of
the string of characters that make it up. Semantically, the optional
comments and FWS surrounding the rest of the characters are not part
of the atom; the atom is only the run of atext characters in an atom,
or the atext and "." characters in a dot-atom.