Hmmmm...........
https://www.google.com/search?=fastmail.com+dnsinspect
give us the january 2018 cached page which contains the same result as now:
Reverse Entries for MX records
OK. All mail servers have reverse DNS entries configured correctly.
Server IP PTR (Reverse) IPs
in1-smtp.messagingengine.com. 66.111.4.72 mx3.messagingengine.com. 66.111.4.72
in1-smtp.messagingengine.com. 66.111.4.75 mx6.messagingengine.com. 66.111.4.75
in1-smtp.messagingengine.com. 66.111.4.73 mx4.messagingengine.com. 66.111.4.73
in1-smtp.messagingengine.com. 66.111.4.74 mx5.messagingengine.com. 66.111.4.74
in1-smtp.messagingengine.com. 66.111.4.71 mx2.messagingengine.com. 66.111.4.71
in1-smtp.messagingengine.com. 66.111.4.70 mx1.messagingengine.com. 66.111.4.70
in2-smtp.messagingengine.com. 185.68.180.24 smx2.messagingengine.com. 185.68.180.24
in2-smtp.messagingengine.com. 185.68.180.20 smx1.messagingengine.com. 185.68.180.20
https://www.maxmind.com/fr/geoip-demo gives US an NL
so... 6 internal MX servers is on a network 1 and the last 2 are on network 2....
one) the 2 networks links collapsed in same time during about 1 hour.... bad luck !
two) if these 8 MX have the same CPU/memory/network links with a weak "margins": if network 1 collapsed more than 18h (6/8 of 24h) with 0 troubles in network 2... lost mail can occurs.