topic Unknown User in HubSpot in Email Deliverability

Unknown User in HubSpot

CAltindas — Thu, 03 Jul 2025 16:38:01 GMT

Hi,

I was just wondering about the reason behind this: An email address is marked as valid in ZeroBounce but appears as a hard bounce in HubSpot with the tag "Unknown User."

I checked, and the address has received marketing emails in the past. Why would it suddenly be marked as an "Unknown User"?

Best Regards
Ceyda

Re: Unknown User in HubSpot

karstenkoehler — Thu, 03 Jul 2025 16:43:48 GMT

Hi @CAltindas,

I see two potential main reasons here:

A bounce occurs at an exact point in time. A HubSpot send might have gotten a response that there is no such email address, an earlier or later validation using ZeroBounce might not have. This could have been due to the email address actually not existing at that time (e.g. recipient was on leave, currently not provisioned an account in their org or similar).
Email service providers don't have any duty to provide accurate response codes and in a company, "Unknown user" might simply be the response that is given to all rejected sends. Since emails from HubSpot and a company come with the "baggage" of a sender reputation and ZeroBounce's validation presumably doesn't, this could also explain the discrepancy. The existence of identity tracking parameters in HubSpot alone could trigger email policies on the receiving end.

In any case, it's hard to troubleshoot retroactively or identify the exact reason without having access to both the detailed logs by ZeroBounce and the settings on the receiving end. Your best option is to simply unbounce the contact if they're showing as valid.

Best regards!

Re: Unknown User in HubSpot

CAltindas — Thu, 03 Jul 2025 16:51:02 GMT

Thanks for your quick response. I wasn’t aware of the point with the accurate response codes within the company, but I’ll unbounce this for now and keep an eye on it.