I see that there's a thread about this already, I created this one separately as it may be a different issue. I've looked extensively into what may be causing this, and I haven't found any errors with my server configuration. This, as far as I can guess, must be the result of some requirement of these providers that I'm not meeting.
I have:
Valid forward and reverse DNS
SPF Records
Static IP address
Not on any public blacklists
The problem is also definitely at the server and not the domain level. The problem occurs on all domains on my server, even one I haven't used yet other than for this test. The headers from messages marked spam also don't provide any useful information. I'm going to figure out how to set up DKIM on my qmail installation and see if that resolves it, but I don't think this is the issue since the hosting company I work for doesn't use DKIM and doesn't have this issue.
I have:
Valid forward and reverse DNS
SPF Records
Static IP address
Not on any public blacklists
The problem is also definitely at the server and not the domain level. The problem occurs on all domains on my server, even one I haven't used yet other than for this test. The headers from messages marked spam also don't provide any useful information. I'm going to figure out how to set up DKIM on my qmail installation and see if that resolves it, but I don't think this is the issue since the hosting company I work for doesn't use DKIM and doesn't have this issue.