Improper folded header

[Simon Brereton]

Hi

Lately I've been getting a lot of these..


/var/spool/mail/quarantine/badh-x9ylzx+MCHZB
---
Return-Path: <jaxbar at example.com>
Delivered-To: bad-header-quarantine
X-Envelope-From: <jaxbar at example.com>
X-Envelope-To: <pj at example.net>
X-Envelope-To-Blocked:
X-Quarantine-ID: <x9ylzx+MCHZB>
X-Amavis-Alert: BAD HEADER SECTION Improper folded header field made up
        entirely of whitespace (char 09 hex): Subject:
        ...IMOgIHVuIGLD?=\n\t=?UTF-8?B?qWLDqSBzaW5nZSAh?=\n\t
X-Spam-Flag: NO


Does anyone know which/what client causes this, and how?  What is the
potential exploitable value of this?  I.e. if I set amavis to ignore
this, what risk do I run (even highly theoretical).

Thanks.

Simon