Users told me they had problems with some online mail clients (like gmx and gmail) viewing mails sent with my MIMEMailxPHP4_V2 PHP class when cid (base64 coded inline) images are contained.
There was a bug initially found by Patrick Kränzlin. This bug is fixed now, you find the current (2.1.3) Version for download (it’s open source) here.
According to the RFC2111:
Note: in Internet mail messages, the addr-spec in a Content-ID [MIME] or Message-ID  header are enclosed in angle brackets (<>). Since addr-spec in a Message-ID or Content-ID might contain characters not allowed within a URL; any such character (including “/”, which is reserved within the “mid” scheme) must be hex-encoded using the %hh escape mechanism in [URL].
I forgot the angle brackets in version 2.1.2
Sorry for that!