Multiple domains getting backed up delivery queue’s at the moment.
Different e-mail servers, suspecting ASG issue.
Non delivery for 30 min approx as of this post.
Emailing Support to advise them.
UPDATE:
Problem appears to have resolve itself, both domain queues functioning normally now.
Hi Ken,
Without wishing to sound facetious (well this time at any rate 
), that’s kinda the point of the delivery queue. If for whatever reason we cannot deliver email, including such reason such as the DNS thing that happened in December (see the below links), such as your ISP having flapping problems or outage problems, your perimeter firewall having issues, your internal networks having issues or your mail-server having issues, then he undeliverable emails are stored in the delivery queue to prevent losing them.
With archiving bolted on, these delivery-queue-stored emails can also be replied to or forwarded from the user’s console.
DNS “thing” links
https://www.schneier.com/blog/archives/2015/12/attack_against_.html
http://www.root-servers.org/news/events-of-20151130.txt
KRs,
M.