Mail

One of the spam crowds blocks a lot of webhosts, puts the server in a blacklist and this list gets spread around. forgot who it is, but should be able to find out who. you can try email your webhost to try ask them to remove their server but all it takes is one complaint from someone about someone on your set of servers and your back on it.
Also AOL has real stupid spam filters set up at the moment

Blacklist may be an idea. I'll check up on that.

The mailer sends a mail only to the client's one email address, so I'm looking to defeat every spam filter in the planet. Just this one. But I suspect if I figure out how to get past this one, I'll break through most others.

How do spam filters work, anyway? I know there are many levels of "defence", but I can't pick out which one is causing this to be blocked. As I mentioned previously, I think it's the mis- (or un-) configured headers -- there's a nobody@localhost and a nobody@[domain] in the from field. Do filters expect anonymous server-generated mails with apparently spoofed headers to be spam? Or am I missing the point altogether?

it was spamcop, here is a quote from them

Quote: SpamCop now implements "pre-emptive" blocking of hosts. This is based on non-SUBE points (mail volume) alone, and is not related to complaints. If a host has no mail volume within the past 7 days except for a 1 day or less period where it does show volume, it will be listed. For example, a host which has no more than 24 hours history for sending mail will be listed under the assumption that it is most likely a new source of spam (since the great majority of new sources of email are sources of spam). After 24 hours, we hope that users will have had a chance to report spam from the new host - or not. If they do, then the other rules will list the host. If they don't (and the host keeps sending mail), then it will drop off the blacklist.

and I believe if you put your ip on the end of this link you should be able to see if your blocked or not www.spamcop.net/w3m?action=checkblock&ip=

might also find this useful too http://www.ecommercetimes.com/perl/story/32009.html

Well, the server is not blacklisted (*phew*) but I'll check with the host if he might be on any other BLs.

did you check sendmail logs (or whatever your mailserver is) logs are the best friend of any mailserver admin

As it happens, I admin very little on that site and server (not even the actual live site), so not much I can do to get my hands on the logs. What should I be looking for in the logs?

Any sign of bounced mail, or other indication as to why the email didn't go through.

No sign, afaik. The local inbox doesn't collect it as trash or spam, so best guess is that a server along the way blocked it.

Usually the server write in the logs the reson it didn't send the message. I would be looking first did the message got all the way to the server then if it did the reason why it didn't go any further.