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Encoded words allow non-US-ASCII characters to be encoded as US-ASCII and thus placed into email headers.

http://en.wikipedia.org/wiki/MIME#Encoded-Word

Encoded words are often used in the Subject header, especially when the Subject is in another language, such as Spanish:

Subject: =?iso-8859-1?Q?=A1Hola,_se=F1or!?=

Question: Is there a legitimate reason to use an encoded word in any header other than the Subject?

Answer:

Examples from RFC 2047

  • Display Names in email addresses: To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?=
  • Comments: From: Nathaniel Borenstein (=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=)

Edit:

I actually received one of these recently. The "guilty" party was Airmail. Here's the headers (with the email address changed):

To: "=?utf-8?Q?john=40doe.org?=" <john@doe.org>
Message-ID: <etPan.534b7be1.6c8b4667.233@myMac.local>
Subject: check this out
X-Mailer: Airmail (237)
6

Human names aren't limited to US-ASCII so anything involving a person's name (e.g. To:, From:', Reply-to: and CC:) are legitimate. RFC 2047 shows examples of this. It wouldn't surprise me if product name headers (X-Mailer:) showed up with encoded characters as well.

  • 1
    Not the local part or the domain, but the display name. Gotcha. – james.garriss Mar 4 '14 at 20:40

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