I occasionally get complaints from people who are subscribed to the weekly DNA Lounge mailing that this is all they see of the messages I send out. The messages are multipart/alternative with text/html and text/plain parts. The HTML part passes the W3C validator,* and all parts are 7 bit and have short lines. I'm as certain as I can be that the messages are valid.
I've heard of it from people using Earthlink, the AOL web-mail client, and once, Thunderbird. (The Thunderbird guy was an AOL user, and managed to forward back to me the message that he couldn't display in Thunderbird -- it was identical to the message I had sent, not mangled, and it displayed fine for me in Thunderbird.) Update: In hindsight, it's likely that all of these people were actually using Earthlink, but were confused.
I'd write this off as "people who use AOL deserve what they get" but it seems more widespread than that, and I've been (very occasionally) getting these complaints for years. Telling them, "Beats me, report it as a bug to Earthlink" produces the predictable cricket-sounds.
If you want an example to test, subscribe to the list, there will be a new one sent out at 6pm on Tuesday. (Giving you a link to the raw HTML or a packed-up mult/alt probably wouldn't be a real test.)
* (And by "passes", I mean to say that the dozens of errors that it spews look like bullshit to me, which is the best that you can ever expect from that useless piece of crap.)
Update: Here we go again. Look, people, all of you who keep piping up with, "Maybe some software (that I don't know the name of) is doing this because it's trying to protect against viruses or something and is screwing up?" No shit! I got that far myself! Your theories about why some shitty software might fuck up this way don't help me. I'm asking what shitty software is doing this, so that I can figure out what it expects so that I can work around it. If all you have are vague guesses, rest assured that I have already formulated those myself.
Update 2: I tried to contact the one guy who seems to be responsible for Earthlink's mailer and he ignored me. I guess they don't care.