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basically they want you to quit spamming … are you sending bulk email? If so have you checked all the things on the recommended list, and signed up for their Complaint Feedback Loop Program? PM me your box hostname if you’d like me to look at what may be the cause.
Ok, well then there is some confusion. WHOIS shows that the NameCheap servers are providing DNS for your domain. You even said that you wanted to use NameCheap’s DNS. So if you want to use the MiaB for the DNS, then you need to change the name servers at NameCheap to ns1.box.firstsightmedia.com and ns2.box.firstsightmedia.com. And add the glue for ns2 if you have not.
There is still no sign of any other records in DNS relating to email. I have dealt with NameCheap’s DNS in the past and from my experience it is horrible. I have never been able to set a record correctly in their DNS as they have specific ways that they want them entered which is not generally overly intuitive. I have always asked them to set the records. Perhaps it is time to contact support for some help setting these records.
I see that you are now publishing DKIM and SPF records in DNS … I presume that the call to NameCheap resolved the issue. Will you please share what the issue was to potentially help others in the future.
Also, now that you are publishing DKIM and SPF (and I assume DMARC records) has your deliverability situation improved yet?
Namecheap appends the naked domain to txt entries. So in the provided DNS admin, when you create the host name DO NOT add the naked domain to the HOST form entry, like in my example .ie firstsightmedia.com