This is going to be a unique issue and not everybody will be affected but it was kind of annoying until I understood what is happening.
I put dnsforge.de as one of the upstream DNS servers.
As a result, nobody could check or sent emails hosted on hostgator.
Reason for this was that DNSforge.de is resolving the mailserver as 127.0.0.1. and that confused the heck out of me. see "dig" below
If you check https://dnsforge.de/ they filter domains using Public blocklists. Hostgator mailserver is not on any of these lists.
I can't find why this particular Hostgator mailserver is on their blocklist, I guess they must have a private blocklist that I can't see.
So I stopped using DNSforge and everything is fine now.
dig @176.9.93.198 mach4.websitewelcome.com
; <<>DiG 9.10.3-P4-Raspbian <<>> @176.9.93.198 mach4.websitewelcome.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13292
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;mach4.websitewelcome.com. IN A
;; ANSWER SECTION:
mach4.websitewelcome.com. 3600 IN A 127.0.0.1
;; Query time: 119 msec
;; SERVER: 176.9.93.198#53(176.9.93.198)
;; WHEN: Wed Jan 13 22:52:59 PST 2021
;; MSG SIZE rcvd: 69
