The short answer: Add us to your safe or trusted senders list and all messages should go through.
The long answer: It depends on your email service what comes through and what doesn't. Email service providers don't announce what they look for in determining what's spam and what's legit because they don't want spammers to circumvent their checks. The developers and I have gone through and implemented "best practices" for email, in terms of what we know email service providers are looking for- meatier content, a custom template, server-side verification and code specifically written to the providers to show that we're legit. Implementing those got us roughly a 50% success rate- in the onset of it, no emails were coming through at all. Heck, some email providers have totally blacklisted us as spammers and we weren't even landing in the junk folders. Some of the things they look for are really obscure, like they watch how many times emails come through from a specific sender to an invalid address. In the programmers' minds, more emails being sent to the wrong place indicates we're using a mailing list and are spam. Unless I start removing email addresses from our system when they bounce (which I can't do because then people couldn't log in), we're always going to have a fair amount of bounces. That's just one example, though. There are lots of things your email provider is looking at.
I had that problem for a while, using Thunderbird mail agent. I finally deleted it and went looking for something else. All else had other worse problems. So after a few weeks I gave up and downloaded Thunderbird again. It turns out they wrote a new version while I was away and now it works just like they knew what they was doing.
Not Sure 