I think you are getting marked as spam, not because of how fast mail is being sent, but because people don’t like the content of the mail. Check yourself, using the URL provided in the screenshot:
http://www.sorbs.net/lookup.shtml?184.173.153.207
Problem Entries, (listings will cause email problems.)
33 “Spam” entries [10:33:37 27 Jan 2017 GMT+00].
184.173.153.207 - 33 entries [10:33:37 27 Jan 2017 GMT+00].10 “Spamvertised” entries [20:21:03 11 Jan 2017 GMT+00].
184.173.153.207 - 10 entries [20:21:03 11 Jan 2017 GMT+00].
See also
https://mxtoolbox.com/SuperTool.aspx?action=blacklist:184.173.153.207&run=problempage`
This is a mailgun IP – unless you pay for a dedicated IP you will suffer due to poisonous neighbors sending actual spam from the same IP address! I struggled with this for weeks and tried to manually de-list the IP with each spam list over and over and it kept getting re-listed. Once I paid for a dedicated MailGun IP I never had a single bounce due to spam. 100% true story. Night and day difference.