In the course of replacing an old Python 2.7 email filter tool with a rewritten Python3 version, I stumbled across a ugly case, where such an header:

X-Microsoft-Exchange-Diagnostics: =?utf-8?B?MTtCTDJQUjAyTUI1MTQ7MjM6bEtRRlNaUHQvVTk5WCttdktlOUVrUGQvVFBH?=
 =?utf-8?B?cDFJemVUeXFzOGNzYnZOYWlwMDZpR0YzbXZyY09WaTBKM2pkeUl4S1VDMkxw?=
 =?utf-8?B?eVRkNWthRW9waUhJTzczTWd5WDZOQ3hMNU1haGFvQTVzVTdRZmxJUnZlblpW?=
 ...

is regenerated as:

X-Microsoft-Exchange-Diagnostics:
 1;BL2PR02MB514;23:lKQFSZPt/U99X+mvKe9EkPd/TPG
p1IzeTyqs8csbvNaip06iGF3mvrcOVi0J3jdyIxKUC2Lp
yTd5kaEopiHIO73MgyX6NCxL5MahaoA5sU7QflIRvenZV

which is plain wrong of course.

I'm using email.message_from_binary_file for parsing and BytesGenerator.flatten for regeneration. Since those are LKML public mails, I'm attaching both versions.

I'm using 3.4.4, but also the email module from current hg for testing.