I’ve got the exact same issue with SparkPost too. Every outbound email is being rejected.
This is the raw json
{"bounce_class":"25","customer_id":"94563","error_code":"550","event_id":"84456312826851433","msg_from":"reply+verp-be6a5b9dba99f405d9de60d7c1925275@fastgrowth.io","raw_rcpt_to":"jake@fastgrowth.io","raw_reason":"550 5.6.0 JSON parsing error in X-MSYS-API: continue at offset 0","rcpt_meta":{},"rcpt_tags":[],"rcpt_to":"jake@fastgrowth.io","reason":"550 5.6.0 JSON parsing error in {hash}: continue at offset 0","remote_addr":"159.203.174.197:52884","tdate":"2016-11-02T22:44:07.000Z","template_id":"smtp_84456312805812268","template_version":"0","transmission_id":"84456312805812268","type":"policy_rejection","timestamp":"2016-11-03T09:44:07.000+11:00"}