David Z's questions

I use Google Apps to handle email for my personal website, so I have an email address [email protected] through that, and I also have a Gmail account [email protected]. Now, I've been trying to send emails to a particular recipient who shall be known as [email protected]. When I send the email from my Gmail account with the @gmail.com address, it works fine. However, when I send it from my Google Apps account with the @ellipsix.net address, I get a bounce message which includes the following text:

Delivery to the following recipient failed permanently:

[email protected]

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 554 554 mail server permanently rejected message (#5.3.0) (state 17).

The bounce message suggests that it is up to the mail administrator of the recipient domain example.com to fix the problem, whatever it is. But I would like to be as sure as possible that nothing needs to be fixed on my end. I already have DKIM signatures enabled for my domain, and I have published an SPF DNS record. Is there something else I should check or do, or can I be confident that it's up to the recipient to fix this issue? Does the "state 17" in the bounce message mean something relevant?

I've included my domain name in the question so people who know more than me about this stuff can independently check the relevant DNS records or other information.

This other question seems similar, but I've already investigated everything suggested in the answers there (except for contacting Google, which I don't want to do unless I suspect it's their issue to fix).

Update: I got a report from allaboutspam.com which shows that by SPF record prevents ellipsix.net from sending email on behalf of ellipsix.net, which seems strange because it's Google's servers (not mine) that are sending email on behalf of my domain; it also shows that my DKIM signature is invalid, which also seems strange because I copied and pasted the DKIM DNS record generated by Google's control panel. I think I have some reading to do...

I have a server running Apache that often gets unwanted proxy requests, mostly for sites like www.yandex.ru or www.baidu.com. I don't have mod_proxy loaded, so these requests don't actually work. But what happens is that the content returned corresponds to the home page for my website, presumably because that's the default virtual host. The thing is, I'd rather not send back the full index page to someone who obviously isn't actually interested in my website; I consider it a waste of bandwidth and resources. I'd rather respond with an HTTP error code (400 I suppose) to signal that my server does not work as a proxy, and also to make myself feel better when I examine the access logs ;-)

What's the recommended way to configure Apache to return an error code for a proxy request? (By "proxy request" I mean one that provides an absolute URL in the HTTP request line.) And which HTTP error code is most appropriate for this?

By the way, I thought this would have been asked already but I looked and didn't find it.