Hi,
Acymailing reports the mail has successfully been send, so it must go wrong on receiving servers.
If you have the Enterprise version open your campaign, go to the tab Tests and run a spam test (marked as 1 in the image). It will likely suggest some improvements.
In case you don't have the Enterprise version you can run the same test manually. Please visit http://www.mail-tester.com/ copy the mail address it gives you and send your test mail to that address (marked as 2 in the image), after which you can see the results on that site.