The normal case works fine:

Mail.build()
|> Mail.put_from({"Tim", "test@example.com"})
|> Mail.render()
|> IO.inspect()
|> Mail.Parsers.RFC2822.parse()
|> IO.inspect()

outputs:

"From: \"Tim\" <test@example.com>\r\n\r\n"
%Mail.Message{
  headers: %{"from" => {"Tim", "test@example.com"}},
  body: "",
  parts: [],
  multipart: false
}

But any non-ASCII causes issues. Changing "Tim" above to "王" outputs:

"From: =?UTF-8?Q?\"=E7=8E=8B\"?= <test@example.com>\r\n\r\n"
%Mail.Message{
  headers: %{"from" => {"?=", "test@example.com"}},
  body: "",
  parts: [],
  multipart: false
}

Note the human readable name is coming out as "?=" with this parser.

The pertinent parts of RFC 2047 §5(3):

In this case the set of characters that may be used in a "Q"-encoded
'encoded-word' is restricted to: <upper and lower case ASCII
letters, decimal digits, "!", "*", "+", "-", "/", "=", and "_"
(underscore, ASCII 95.)>.

These are the ONLY locations where an 'encoded-word' may appear. In
particular:

• An 'encoded-word' MUST NOT appear within a 'quoted-string'.

Essentially, there shouldn't be a " within the encoded-word and the encoded-word can't be put within a quoted-string.

In this case, the proper encoding would be From: =?UTF-8?Q?=E7=8E=8B?= <test@example.com> and the parser properly returns headers: %{"from" => {"王", "test@example.com"}}