So
SSH into your server and do this:
ping smtp.sparkpostmail.com
Also, if you:
cat /var/discourse/containers/app.yml | grep "smtp.sparkpostmail.com"
What’s the response?
If you don’t get back a line like this:
DISCOURSE_SMTP_ADDRESS: smtp.sparkpostmail.com
It’s very likely that you typo’d the address during discourse-setup, which would explain why your server can’t resolve it. You can re-run discourse-setup or edit the app.yml to correct it.