A JSON parsing error in X-MSYS-API is causing email failures for users of the SparkPost mail provider. philh initially reported the issue, and JakeHower shared a raw JSON output showing the error. The issue was specific to SparkPost and related to a header used to send the message_id in case of a bounce zogstrip. A fix has since been implemented, as announced by zogstrip, and is available in this commit.
On two different relatively new instances email has stopped working.
Sending test email from admin shows this in admin/email/skipped
[Sender] 550 5.6.0 JSON parsing error in X-MSYS-API: continue at offset 0
Tried rebuilding but no change. Any suggestions?
Edit: running 1.7.0.beta7
Are you using sparkpost as mail provider?
Yes, using sparkpost.
Hmm so this is specific to sparkpost?
Yeah. It’s the header we use to send SparkPost the message_id in case we get a bounce.
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"}
This should be fixed now 
https://github.com/discourse/discourse/commit/17169b3037037103a54317c1f03b86f1359ed1a7