[through]

Good question. I do not know but I would guess that your domain hosting company and the range of IP addresses it uses could be blacklisted, or, the email provider has a well known data signature that vendors associate with spam bots and again, blacklisted. I would be interested to know myself. Best guess; perhaps change email provider or hosting company and see if situation can be replicated. Ensure your email addresses have not been compromised. Good luck.

[jazzkin]

You're making some valid points and I could try to do it trough my own servers or some other provider but it feels like Fastmail should be fairly reputable but I could be wrong.