I have this issue with some of my clients as well.
Here is an article with the IP addresses your tech department should whitelist: https://support.cvent.com/s/communityarticle/What-Cvent-IP-addresses-should-we-whitelist
That article also links to a list of AWS IP's that you may want to whitelist as well.
And here is a article with a list of domains that should be whitelisted: https://support.cvent.com/s/communityarticle/What-domains-should-we-whitelist-for-Cvent-emails
This won't solve the issue all the time, but it can help the situation.
------------------------------
Tori Martinez, Cvent Certified
Event Technology Specialist
Direct Travel
United States
------------------------------