seelystreet.com Domain is on fasthosts The DNS records here point to Netlify which resolves lucid-goldwasser-c97dbd.netlify.com to seelystreet.com the github account seelystreet has two repos one seelystreet and the other seelystreet.github.io this is a submodule which is required for hugo to build and deploy the site. However in this case netlify takes care of building and deploying.
Log into Netlify using github the email for both is computer@fastmail.com
DNS records fm1._domainkey.seelystreet.com 3600 IN CNAME fm1.seelystreet.com.dkim.fmhosted.com fm2._domainkey.seelystreet.com 3600 IN CNAME fm2.seelystreet.com.dkim.fmhosted.com fm3._domainkey.seelystreet.com 3600 IN CNAME fm3.seelystreet.com.dkim.fmhosted.com seelystreet.com 3600 IN MX 10 in1-smtp.messagingengine.com seelystreet.com 3600 IN MX 20 in2-smtp.messagingengine.com seelystreet.com 3600 IN NETLIFY lucid-goldwasser-c97dbd.netlify.com www.seelystreet.com 3600 IN NETLIFY lucid-goldwasser-c97dbd.netlify.com seelystreet.com 3600 IN TXT v=spf1 include:spf.messagingengine.com ?all
Fastmail needs the seelystreet.com domain added to the dns section and it should pickup the MX records from netlify.
An Alias needs adding which will be garth@seelystreet.com pointing to garth39fastmail.com