As you might know, Google, Microsoft and others do not allow sending massive amount of emails from a single account.
In my case my servers send around 800 emails a day and growing. Many of those were blocked and did not arrive to their destination.
The solution I've found was to open an account with a company that gives such service of sending emails masively.
There are many in the market but I want to recommend the one I've found (I'm not working there and do not have any relation with them apart of using their service) - www.mailgun.com
The advantages I recognize are:
1. Price - first 10K emails of every month for free, the next 10K 5US$ and going down as you grow. No one that I have found gives you monthly free emails.
2. Easiness of setting up - it took me literly 5 min to open the account and test with GGS (no credit card needed), including adjusting my DNS records (all explained on the opening process).
3. The company is of Rackspace - where I hold my servers. If mailgun's service is anywhere near of Rackspace's then I can promise you that they give the best service possible, world class level.
4. They give statistics + detalied log were you can see the status of each email including the failure reason (if) and suggestions for solution.
I hope this post will help any of you.