[gvb]

Have you implemented DKIM/SPF/DMARC? If you have not, that is a first step.

Are you sending from a "consumer" network IP block (cable modem or DSL)? If so, that's going to be big points off (probably not since you are sending through fastmail).

In my limited and somewhat dated experience, some email service providers are very aggressive in blocking email senders. The better class of these had contact information and would respond to requests to be whitelisted. Some email providers blocked emails and had no contact information to request white-listing. For them, my only recourse was to send emails to them through an email forwarding service (I used Dyn at the time, Dyn apparently sold their email forwarding business to DuoCircle).

https://en.wikipedia.org/wiki/DMARC

https://en.wikipedia.org/wiki/Sender_Policy_Framework

https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail

[bks]

Brad from DuoCircle here, if you need any help with sending let me know we have a few services that can help. We can smarthost your SMTP traffic on the outbound or we can forward your inbound email to the right destination.

My $.02 on the conversation is to double check your IP on some of the RBL services http://www.anti-abuse.org/multi-rbl-check/ to check to see if you are listed for any reason.

And then send a test email to https://www.mail-tester.com/ and check their scores. You SHOULD be able to get a 10/10 with a blank email as long as your authentication aligns.

Feel free to email me and I would be happy to help.

[jazzkin]

I will have to look through what settings Fastmail are using but this is a good starting of point. It's not from a "consumer" network IP block but I will look into the details that you have provided and see if it changes anything.

Thanks a lot, always good to have a starting point.