[tauntz]

Exactly. That's why you don't want to use these free open relays, ever, for anything that you actually want to get delivered. There are a lot of companies out there that specialize in SMS delivery and have built really smart/complex route selection, retry, failure handling and cost optimization logic to get to a reasonably high delivery %.

/spam: my friend runs https://messente.com that specializes in global SMS delivery and they are, honestly, quite good at it. (Let me know if you ever start using them so I can let him buy me a referral-beer at the pub)