An Unknown User is an email address that does not exist. This is determined after our MTA (Mail Transfer Agent) attempts to deliver a message but receives a 550 ‘User does not Exist’ error code from the receiving MUA (Mail User Agent). This is defined in the Revinate product as a Hard Bounce and is suppressed from future sends. It is best practice to remove all Hard Bounce messages from a database to ensure that the user does not accidentally reintroduce these users back into a future marketing campaign.
The Risks of Sending to Unknown Users:
There are a few major risks with sending to unknown users. One is Internet Service Providers (ISP) track unknown user rates to their system (Attempted Sends) and use that percentage in the senders overall email reputation. If the percentage is bad enough, or if your other reputation metrics are not good, the sender can expect to see delivery issues in the form of a Reputation Soft Bounce.
The second risk is that there’s a chance that the address was converted into a Spam Trap (Recycled Spam Trap). Once converted, ISPs use these Spam Traps to catch senders who are not following industry best practices in the space. Depending on the number of Spam Traps you his, the impact could result in poor deliverability metrics at that ISP & the possibility of landing on a Blacklist.