Where you saw it: A bounce message from your sending platform, a mail log entry, or a verification result marking the address as invalid.
Why it happens
- The person left the organization and the mailbox was deleted rather than forwarded.
- A typo in the address, either from a signup form or from manual entry.
- The address was fabricated at a signup form by someone who wanted the download and not the mail.
- The mailbox was deleted during a platform migration and never recreated.
- The address came from a purchased list and never existed anywhere.
The fix, in order
Suppress it immediately and permanently
A 5.1.1 is a definitive answer. Every serious platform suppresses on the first occurrence, and mailing it again is a direct reputation cost with no possible upside.
Look for a typo you can rescue
Check for gmial.com, hotmial.com, a missing letter in the domain, or a transposed pair in the local part. Correcting an obvious typo recovers a real contact rather than losing one.
Measure the rate rather than the raw count
Under 2% on an engaged list is normal decay. Above 5% means the list source is the problem and cleaning the symptoms will not fix it.
Find out where they came from
Group the bounces by signup source, campaign or import batch. Hard bounces cluster, and the cluster names the bad source.
Verify before the next import
Checking an address before it enters the list moves this failure off your sending reputation and into a report. It is the only place in this list where verification is the direct answer.
550 5.1.1 The email account that you tried to reach does not existGmail's wording. The 5.1.1 code is the part that matters.
How to know it worked
The short version
- The address is in your suppression list and future sends skip it.
- Hard bounce rate on the next campaign falls below 2%.
- No repeat bounces from addresses that already returned 5.1.1.
Questions people ask
Can a 5.1.1 address ever start working again?
Occasionally, when an employee returns or an alias is recreated. It is rare enough that suppressing permanently is the right default, and re-verifying an old suppression list before a major campaign is reasonable.
What hard bounce rate is acceptable?
Under 2% on a list you built yourself. Above 5% and mailbox providers start treating you as a sender who does not maintain a list.
Does verification catch all of these before a send?
Most of them, on domains that answer honestly. On catch-all domains, Yahoo, and servers that refuse to answer, we cannot confirm the mailbox and we say so rather than guessing.