There appears to be a DNSSEC issue with reverse DNS records on 18.104.22.168/8, so if your IP lands in this range (22.214.171.124-126.96.36.199), you’re probably going to find yourself unable to send much email today.
The issue is DNSSEC related, so this will only impact receivers who use DNSSEC resolvers, but as Word To The Wise points out, this is nearly every major mail server today.
Hopefully arin.net is working on the problem, but due to TTLs and caching, DNS issues can linger for a couple of days.
As a sender, what can you do? Well, mostly, nothing but wait it out. As a receiver, you could exempt rDNS checks from this IP range, or just make sure that your mail server returns a temporary (4xx) failure and not a permanent failure for DNS errors on reverse DNS queries, such that mail queues up and is re-delivered rather than refused.