One or more of your mail servers isn’t implementing SMTP correctly. Some do, but some don’t. That explains why some of your messages get through to me, and some don’t; but it doesn’t explain why you send mail differently from different parts of the site. Here’s an article that explains it pretty well:
Short answer: you have an SMTP sender that looks/acts like a spammer. Besides being out of conformance, it’s probably responsible for plenty of other problems. This is the first time I’ve seen a non-spammer using this technique (“slamming”) but even if it saves you some CPU cycles, it’s probably not a great idea.