C-T-E: base64 and the real world.. what should an MUA do?

(Sorry I've been silent on this list for a while, I've been off doing other
stuff for the nonce and only posting if something egregious happened to
catch my eye...)

OK, so the following mail showed up on the linux-kernel mailing list (and it's
not a singleton mail, I've seen this same broken behavior a lot lately...)

It had 1 bodypart, correctly tagged as

Content-type: text/plain; charset=UTF-8
Content-transfer-encoding: base64

Majordomo then goes and sticks some stuff on the end, so the last few lines
of the message are:

--- cut here ---
IGRldmljZV9yZW1vdmVfZmlsZShkZXYsICZkZXZ0X2F0dHIpOwo+ICAgICAgICAgfQo+ICAgICAg
ICAgaWYgKGRldi0+Y2xhc3MpIHsKPgo+ICAtLQoKQXBwbHlpbmcgdGhpcyBwYXRjaCB3b3JrZWQg
Zm9yIG1lLgoKVGhhbmtzIQogICAgICBNaWxlcwo=
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--- cut here ---

Of course, the last 5 lines are bogus as base64, and it's easy to just say
"Majordomo is at fault".  But such things happen in the real world, and what's
the recommended algorithm for an MUA to recover from this?

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (GNU/Linux)
Comment: Exmh version 2.5 07/13/2001

iD8DBQFIDUdbcC3lWbTT17ARAtXdAJ9trfbOzvVx2eN7ZW+dkpnODFY1agCdEHJq
ZqrakaItx2A8LG+N3J6TMmg=
=U6ao
-----END PGP SIGNATURE-----