There are a number of things you can do to identify and remove unknown users from your email list. You can:
- Remove addresses that return a hard bounce. Most unknown user bounces have a 550 error code.
- Monitor your data and look for hot spots of unknown user rates by mailbox provider, campaign, or customer segment.
- Monitor the unknown user rate of your IP addresses.
- Run your list through an email validation tool.
- Ask your subscribers to confirm their email addresses.
- Remove inactive subscribers. Dormant accounts are often deactivated by a mailbox provider and may be turned into a spam trap.