If anyone using my personal domain sends emails to Spark users, they get a NDR as below :

I'm sorry to have to inform you that your message could not be delivered to
one or more recipients. It's attached below.

<user@domain>: host mx1.sparkbusinessmail.co.nz[122.56.84.33] said:
550-5.7.1 Email rejected due to SPF unauthorised mail sending host.
Please
550 5.7.1 refer to
http://www.spark.co.nz/help/internet-data/webservices/sparkbusinessmail/
(in reply to end of DATA command)

I'm also seeing this on a lot of other domains as well (sending to Spark accounts) - Spark have said "not our problem". Yet it only started happening after they switched to SMX. 

All other emails go out fine and arrive at their destinations without an issue.

SPF records are all fine - yet if we remove the SPF record, the email goes through. 

Anyone else seeing this issue ?