How can I send mass emails and SMS to our customers without our domain being marked as spam?

Most spam filters use a learning algorithm. So your domain will be filtered as spam if your other customers are marking them as spam. So if you don't annoy your cusomters with your emails then there won't be a problem.

a couple of things: domains aren't blacklisted, ip addresses are blacklisted. individual recipients and spam filtering products\gateways may mark the sender or the senders domain as spam, but that only affects that particular recipient and has no bearing on any other recipient. there's a difference between unsolicited commercial email and sending billing statements via email to your customers. many companies utilize email to communicate with their customers with little or no problem. microsoft sends millions of emails every day, likely with very few problems regarding being marked as spam. you might block their email to you but it has no bearing on their email to me. there's nothing you can do to guarantee that your mail server or your email will never be blacklisted or caught by someone's spam filter. from a technical perspective, any and every email you send could get your mail server blacklisted. if every company had to worry about every single email they sent for fear of being blacklisted or filtered, no-one would ever use email ever again. how many customers do you have? a couple hundred emails a month going to a couple hundred different email addresses doesn't constitute mass emailing imho. make sure to give your customers a way to opt out of these emails and respect their request if they do opt out.

You can adopt some of the best practices and make your Email look less SPAMmy. To check your server against some of the best practices, try the Online test at http://www.allaboutspam.com/email-server-test