553 5.1.3: Recipient address is not valid
553 5.1.3 means the address is written wrongly, so the server rejected it before looking for a mailbox. The problem is the address format, not the person. Clean the address and send again.
- Bounce type
- Hard bounce (permanent)
- Usually caused on
- Your sending side
- Terlido report label
- Invalid recipient
What the standard says
RFC 5321 defines 553 as "mailbox name not allowed", for example when the syntax is incorrect. RFC 3463 defines 5.1.3 as "Bad destination mailbox address syntax": the destination address was syntactically invalid. The server never tried to deliver. It refused the address as written.
What the mailbox providers say
- Gmail (553 5.1.3): “The recipient address <address> is not a valid RFC 5321 address...” Source
Common causes
- A space inside the address or a space left at the start or end of the cell.
- Two @ signs, often from two addresses pasted into one cell.
- A trailing dot, a trailing comma, or a semicolon left over from a list that held several addresses.
- Smart quotes or hidden characters that came from a CSV exported from a spreadsheet or a web page.
- An address in the form Name <email> where the whole text was used as the address instead of only the part inside the brackets.
- A mail merge or import that put the wrong column into the email field, such as a name, a phone number, or a website.
How to fix it
- Trim spaces and strip stray punctuation from the email column before importing.
- Split cells that hold more than one address into separate rows.
- Re-export the CSV as plain UTF-8 and remove smart quotes and invisible characters.
- Check the import preview and look at the first rows, because a column shift often turns names into addresses.
- Run the list through a format check before you add it to a campaign, and fix every row it flags.
- Keep one address per cell and one cell per contact, so the importer never has to guess where an address ends.
Is it a deliverability problem?
A syntax error is cheap to prevent and costly to ignore. It says the list went into the campaign without checks, and a pattern of them can push the hard bounce rate up. The fix is on your side and takes minutes: validate the format of every address before you launch. This error also tells you that the contact never had a chance to read your message, which wastes the work you put into the first email. Catching syntax errors costs nothing and does not need a send test, so there is no good reason to let them through to a live campaign.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 5 is a hard bounce. In the campaign Reports tab this one is grouped as "Invalid recipient".
On a hard bounce Terlido adds the address to the workspace blocklist with the reason "hard bounce", so no campaign emails it again. The lead stops in that campaign as bounced, its lead score goes down by 15, and the bounce counts toward the campaign's bounce rate.
Once a campaign has at least 20 sends, it pauses itself when hard bounces divided by sends goes over the campaign's maximum bounce rate. The default is 5%. If the cause was on your side and you fixed it, you can remove the entry on the Blocklist page of your account settings and try again.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Email list format checkerReview CSV structure, empty addresses, and basic email syntax without contacting mail servers.
- CSV campaign import checkerCheck mapped email and personalization columns before import.
- Text cleaner & counterClean extra spaces, remove repeated lines, change case and count words or characters. Your text stays in this browser.