Most of the time, when block issues arise, they have to do with the content of your messages. Internet Service Providers use their own spam filtering techniques and will sometimes block messages they consider as spam. The best thing to do is try to change the wording of your message. You'll find a lot of resources on the web on this topic, like this article.
On rare occasion, blocked messages might have to do with IP issues on our side. We work closely with Internet Service Providers all around the world to avoid it, but it remains a possibility. If you spot in your logs that we have trouble with a particular provider in your country, please log a ticket to let us know.