This solution is related to the "This message has been classified as spam. If this is a false positive please open a support ticket referencing message" message. If you are receiving this message your email has been deferred and was not accepted by our email server.
As of September 2018, we reconfigured our SMTP server to no longer relay any messages which our anti-spam software reports as "definitive spam" (100).
This is a change in our message handling process. Historically we permitted up to 15 spam messages per 8 hours, and established customers up to 50 per 8 hours because we were not confident in the detection from accuracy from the system. However, after extensive testing, we are now greater than 99% positive that messages reported as (100) should not be accepted.
Email fitting this criteria will be soft-failed, and your sending (script, email client, mail server) software should notify you the message was not accepted by our servers. We cannot account for how each mail server or email client will display this error, but your safest bet is to review your logs and outbound messages queues. If there is email stuck in the queue - this could be the reason.
Effective January 1st, 2019 our policy will be to reject this email.
This brings our outbound anti-spam policy on Outbound in closer alignment with our policy in Inbound, which has always been to reject mail receiving this classification.
Our spam filtering engine is not immune to false-positives, so there is a chance that your message was misclassified as spam, in instances where you feel that it should be classified as legitimate email. In these instances please contact technical support and they have the experience to help you solve your deliverability issues.