Why Your Subject Line Arrives as =?UTF-8?B? and a String of Letters

Why two encodings, and which you get

Q leaves ordinary letters alone and escapes only what it must, so a subject that is mostly English with one accent stays almost readable in the raw file. B encodes everything, which is shorter when most of the characters are non-ASCII — a subject in Greek or Japanese is far more compact as base64 than as a string of escapes. Mail programs choose per header, which is why one message can carry a Q-encoded subject and a B-encoded sender name, and why both have to be handled.

What plain text is good for

A message reduced to text is searchable by every tool on your machine, quotable without dragging a layout along, and small enough to keep by the thousand. It is also the form that survives: an HTML message from 2011 renders differently in every reader that has existed since, while its text alternative reads today exactly as it read then. Most messages carry both, and the text one is the one nobody looks at until they need it.

Frequently asked questions

My subject shows as Réunion instead of Réunion. What is that?

The opposite problem: the bytes were decoded, but with the wrong charset. é is what the two bytes of a UTF-8 é look like when read as Latin-1. It usually comes from a program that ignored the charset the header declared. Opening the original .eml and letting the tool read the declaration gives the right characters back — the file was never damaged, only misread.

Does the text version lose anything?

The layout, the images and the links' destinations — a link becomes its words, not its address, unless the sender wrote the address out. What it keeps is everything that was said. If you need the addresses too, take the HTML conversion or read the message in the viewer, where the links are still links.

Can I convert a whole mailbox at once?

Split the .mbox into individual .eml files first, then convert the ones you want. That two-step route is deliberate: a mailbox is usually far larger than the handful of messages you actually need as text, and splitting first lets you pick.