RFC 5322 allows special characters, including ? and =, to be used in e-mail addresses.
RFC 2047 is an extension that adds a feature called "encoded words" which let you embed different encodings in the same header. However, it explicitly says that these aren't allowed in e-mail address headers.
Encoded words have the format:
encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
Where encoding is either Q or B, but could take on other values in the future.
After this change we consider e-mail addresses with an encoded word inside invalid.
We validate the *format* of email addresses in many places with a match against
a regex, often with very slightly different syntax.
Adding a separate EmailAddressValidator simplifies the code in a few spots and
feels cleaner.
Deprecated the old location in case someone is using it in a plugin.
No functionality change is in this commit.
Note: the regex used at the moment does not support using address literals, e.g.:
* localpart@[192.168.0.1]
* localpart@[2001:db8::1]
