High number of hard bounces (> 4%)
Sending to an old or bad list
Sending to an opt-out email list
Mailbox provider is deactivating old, inactive email addresses
An unknown user error code is generated when a sender deploys an email to an email address that no longer exists. This could be because the email address never existed, is no longer active by choice, or was abandoned by the end user.
When mailbox providers see a high unknown user rate from an IP address, they may suspect a "dictionary attack" is occurring or that the sender does not have solid data hygiene practices. A dictionary attack is when a spammer uses combinations of words in the dictionary to create random combinations of emails addresses and then deploys spam to those addresses for the purpose of finding valid email addresses. These addresses are then collected, sold and sent more spam. To prevent more unknown users, you should:
Verify that your optin processing system is handling verification of email addresses. We recommend you use the list hygiene service built into Emercurythat removes spam traps and hard bounces or you may use a third party email verification service before sending.
Mail to all of your email users no less than once a quarter.
Mail a sample of your lists before mailing to the full list to measure your ratio.
If you find your hard bounce ratio is too high inquire about email verification service from your account manager.