It is problem with IP

barsa — Mon, 16 Mar 2009 07:21:57 +0000

It is problem with IP address where your website is host.
May be Yahoo and Hotmal ban this IP address so your email goes spam folder.

I had a client with the

decibel.places — Fri, 27 Feb 2009 04:26:10 +0000

I had a client with the domain "ratemybutt.com"

We sanitized the from email address, but the underlying domain name was still in the mail header - probably not related to your problem, but an observation...

It's not the domain name -

pr0gr4mm3r — Fri, 27 Feb 2009 04:21:41 +0000

It's not the domain name - it is likely the IP address of your server. Try checking your IP address here & here and see what you find.