It's not the email library but your MX domain, mainly the fact you are missing some things. You have to setup DomainKeys / DKIM and SPF in order for your emails to be legit. And, even after you setup these, don't expect to go inbox, it will be 50/50 chances for this to happen. Email is a real nightmare nowadays.
The only reason why you go inbox right now is because your ip address is clean and did no spam, but don't expect this to last, you'll get blacklisted sooner or later.
Just a hint, you can use smth like http://mandrill.com/ (they have a nice api) for sending emails, it is free for small amount of emails and can get you going pretty fast, mostly the important thing is that you will reach the inbox everytime and you don't have to go through the nightmare called email setup.