If you or your colleagues aren't receiving emails from Halcyon, the cause is almost always a security layer on your company's mail system holding the message before it reaches your inbox.

Halcyon sends email from three different systems, depending on the type:

All three are diagnosed the same way. The steps below apply to any of them; the only difference is which sender and IP ranges your IT team allowlists, covered in step 4.

On our side, our system shows these emails as Sent and Delivered to your address. "Delivered" means a mail server on your end accepted the message and confirmed receipt back to us — so the email is being accepted somewhere in your environment, it just isn't reaching your inbox.

When a message is accepted but never lands, and it also doesn't show up in your mail platform's trace or quarantine (Microsoft 365 or Google Workspace), that usually points to an email security layer in front of your mailboxes that is intercepting and holding it.

Share these steps with whoever manages email security at your organization.

Look at any gateway that filters mail before it reaches Microsoft 365 or Google Workspace — common ones include Proofpoint, Mimecast, Barracuda, and Cisco IronPort. The message is most likely being held here, not in 365 or Gmail itself. Search that system for the relevant Halcyon sender (see step 4).

Microsoft 365: In Microsoft Defender, review the Tenant Allow/Block List and the Defender quarantine (under Rules & Policies). Add the Halcyon sender address(es) and the halcyon.io domain as allowed.

Google Workspace: In the Google Admin console, under Apps > Google Workspace > Gmail, check the Admin quarantine for held messages. Add the Halcyon sender address(es) and halcyon.io to Approved senders (in the Spam, phishing, and malware settings), and add the sending IP ranges to the Email allowlist. Also have the recipient check their personal Gmail Spam folder and mark our messages as "Not spam."

Microsoft 365: Use message trace in the Exchange admin center.

Google Workspace: Use Email Log Search in the Google Admin console (Reporting > Email Log Search).

In either tool, filter on the exact subject of the email you're expecting and the matching From address, scoped to the time window when the email was sent.

Allowlist the From address and, where applicable, the sending IP ranges for whichever email path you're missing. If you're not sure which, allowlist all three.

From address: platform@halcyon.io

For emails sent from Hubspot, the From address and the halcyon.io domain are the primary control. As a secondary aid, your Halcyon contact can provide a list of Hubspot IP addresses currently in use for our account. However, these IP addresses are shared pools used by many senders, and they change over time, so allowlisting them alone trusts more than just Halcyon.

From address: alerts@halcyon.io

To view Postmark’s current IP list, please visit this link and reference provided context before adding firewall rules.

From address: noreply@halcyon.io

For this path, allowlist by sender address and the halcyon.io domain, not by IP. Auth0's email delivery IPs are dynamic and not reliably allowlistable at the network level, so a domain/sender allow rule is the right approach. (If you need an IP-based rule, contact your Halcyon representative.

These articles lay out the allowlisting steps in more detail:

If your IT team has completed these steps and you're still not seeing our emails, reply to your Halcyon contact and we'll re-send and help trace it from our end.