Invalid email address
An address that cannot receive mail, because it is malformed, its domain does not exist or accept mail, or the mailbox does not exist. Sending to it causes a hard bounce.
What it means
Addresses fail at different levels. Some break the syntax RFC 5321 defines for a mailbox. Some have a domain that does not exist or publishes no way to receive mail. Most commonly the domain is fine but the mailbox is not: M3AAWG notes that user unknown is the most common permanent failure, reported with a 5xx code that tells the sender not to retry.
Providers count these failures against the sender. M3AAWG says the volume of permanent failures is one indicator receivers use to build a sender's reputation, and that large volumes usually point to a poor registration process or an old list. Microsoft's high-volume sender recommendations include removing invalid addresses regularly.
Suppress an address after a user-unknown bounce and never mail it again.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- M3AAWG Sender Best Common Practices, version 4.0 (August 2026)m3aawg.org
- RFC 5321: Simple Mail Transfer Protocolrfc-editor.org
- Microsoft Tech Community: Outlook's requirements for high-volume senderstechcommunity.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes