There are two IP addresses used by the VCD for outgoing email. One is general and covers all outgoing messages including conference invites and notifications of messages and meetings. The other is for our password reset emails, and is set up to a different IP to make sure that they always come through.
If you are experiencing issues with a custom whitelabel email address email deliverability internally, we usually see that these emails are being rejected by firewalls attempting to prevent the spoofing of your company URL. These issues rarely impact users outside of the company managing the event (to whom the domain belongs). To resolve these issues, we recommend that you ask your IT department to whitelist the following IP addresses:
Password reset: 18.104.22.168