I got problem with SNS today while changing license domain. After checking firewall and several emails to Sendy support with no luck, I finally found the problem for my case. I report the solution here in case somebody having the same problem.
You should check following modules and firewall:
- curl
- php5-curl (this also for the case your site is not shown)
- not sure (php5-imap)
- firewall
Regards