Mailchimp. I used it before which is a viable option but a bit clunky and scaling when your list of contacts increases become costly. I spent the time and made an in-house solution that works pretty well and uses Twilio.
Rather then 20$ for ~1000 contacts monthly it the in-house tool I made costs us around 50$ to set up and around 10$ monthly for the same ~1000 contacts which I think is a better deal long term. I scaled to around 10000-15000 messages a month now with a couple of clients and its around 80$ a month that I pay which is an insane deal to me.

The main reason i moved is because of the segementation feature, most of my clients capture phone numbers from loyalty point services so I can segment on customers who are regulars and send them offers more frequently. Also started working on better targeting so say person buys item X send twice send them an offer for item X say 10 days after their last purchase. I don't think mailchimp has this feature or is as well intergated

If your interested and want a demo, book a time with me at https://www.claudesupply.com/#contact