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But I can't seem to get past yahoo's spam blocker. Is there a best practice for php mail? I've done a bunch of googling on this but can't find anything definitive.
This is out of the realm of PHP coding. You'll need to research how to configure your mail server (or ask your webhost) to employ signing technologies such as DomainKeys/DKIM.
I can't be sure, but I suspect it has something to do with strange "from" or "reply-to" headers.
Check that this all looks normal. Also, if you send one to me at omniuni@inbox.com I can check to see if it goes into my Spam folder to try to narrow down how/why Yahoo is detecting Spam.
Not a real solution, but keep in mind that Yahoo's spam filtering is extremely crappy
It generates LOTS of false positives (regarding proper mail as spam), so I doubt if you can fix this 100% at your end. Please complain to Yahoo about this, the more complaints they receive, the more likely they will do something about it some day.