mbox can't be implemented properly, it is an inherently broken format. Or rather, a family of almost compatible broken formats.

https://en.wikipedia.org/wiki/Mbox

Sounds like Mail.app exported in mboxcl2, which breaks mail readers expecting the mboxo or the mboxrd formats.

I recommend people to use IMAP for extracting the mail, and to store it in something outside the mbox family.

Takeout engineer here. We currently use mboxo, but will be moving to mboxrd shortly.

That seems quite reasonable. Thanks for the info.

How is mbox a broken format? It's a bad idea to use it in a multi-writer environment, and it's horrible because there are competing versions of it. But for plain storage, I thought it worked quite well.

What am I missing?

Any line following a blank line and beginning with the four characters "From" marks the start of a new message. The BSD way of handling such lines in the body of a message was to prefix them with ">", which is a non-reversible workaround and is in the uuencode target charset.

Prefixing every line in the message body with a tab character would allow reversing and not interfere with uuencode/base64&c.

I've got mbsync set up to periodically dump my Gmail into a local Maildir archive. Works great, and it was a (relative) breeze to integrate with my old mail archives reaching back to the late 90's.

> Can anyone confirm that they've implemented this properly

Well, they are the largest e-mail service provider in the world. My guess is somebody checked the format for bugs.