As Carl mentioned, asking to have Cvent IP addresses whitelisted will help - here's an article that helps with that process and info on allowlisting:
https://support.cvent.com/s/communityarticle/What-is-whitelisting?is_article=true&lang=en_US&searchFor=allowlist&page=1
Also, with the increase of phishing attempts, many companies have increased their email validation processes. One in particular, Domain-based Message Authentication, Reporting and Conformance (DMARC) protocol verifies if the "from" email address matches the domain where the email is sent from. If they don't match, it may prevent emails from entering through the firewalls.
Here's a Knowledge Base article that explains it further and it might be worth checking into if you are finding that there are big issues with communications making it to the intended audiences:
https://support.cvent.com/s/communityarticle/INTERNAL-What-is-DMARC?is_article=true&lang=en_US&searchFor=dmarc&page=1
Hope this helps!
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Rebecca Yousif
Senior Administrator, Event Technology
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Original Message:
Sent: 02-08-2023 17:28
From: Carl Sandstrom
Subject: Increase in attendees not receiving confirmation emails
We have had to ask some of our receiving organizations to whitelist the Cvent IP addresses. That has cured over 90% of our non-delivery issues.
Carl