Quoted-Printable
Email's way of keeping 7-bit safe.
Where it came from
Quoted-Printable (QP) was defined in RFC 2045 as part of MIME, the standard that taught email to carry attachments and non-ASCII text. The design goal was clever: leave ordinary ASCII mostly readable while escaping the rest.
That is why a QP email is still legible in a raw "View Source" — unlike Base64, where everything turns to gibberish.
How it works
Printable ASCII passes through unchanged; anything else is encoded as =XX (two hex digits, e.g. =E2=82=AC for €). An "=" at line end is a "soft break" continuing the line.
Lines are capped at 76 characters with a soft break. UWC encodes and decodes QP entirely in your browser — useful when inspecting raw email.