In a few of my apps, I'm using a simple SMTP server to send reminder emails & text to various people. It works great & everything is fine but we just signed a new client where there will be hundreds of employees that we will sending emails. I was wondering what's available for me to setup a mass emailing like this as I believe the email provider won't particularly like/support this. Should I look at constant contact or maybe mail chimp? What do you guys use out in the wild for something like this. Just trying to get some info to see where to go.

Thanks in advance,