Quoted-printable
An email content-encoding for non-ASCII characters — usually invisible, but matters when it leaks into the body.
Quoted-printable is a content-transfer encoding used in email to safely transmit non-ASCII characters (accents, emoji, smart quotes) through SMTP, which historically only handled 7-bit ASCII. Characters like é are encoded as =E9; spaces at line breaks become =20. The encoding is invisible to recipients when properly decoded.
Quoted-printable becomes visible — as raw =XX codes in the email body — when something in the pipeline fails to decode it. The most common cause is a misconfigured Content-Transfer-Encoding header, or a content pipeline that double-encodes the body. Receivers see strings like "It=92s a great day" instead of "It's a great day."
For email senders, quoted-printable is something to monitor in client-render testing. Modern ESPs handle it correctly by default; custom sending infrastructure is where it tends to leak. If you see =XX patterns in rendered email, the fix is at the encoding layer, not the content layer.