Symbol Security uses a list of IPs that need to be whitelisted in order to bypass some of the phishing simulation emails sent on your campaigns. As an admin you need to ensure that whitelisted IPs are correctly configured.
If you have not whitelisted the Symbol Security IPs or domains probably emails won't be delivered or will be directly sent to the spam folder or quarantine. Click on the following article If you want to see the list of IPs and domains used by Symbol:
If you don't know how to whitelist it, here there are some instructions about how to do it depending on your vendor:
If your company have some firewall, add the IP to the whitelisting or exclusion list as well.
S Symbol is the author of this solution article.