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553 5.1.3 error: invalid email address format

553 5.1.3 means the recipient address isn't a valid email address at all: a stray character, a missing @ or a broken import. Fix it or remove it.

List problemThe address is the problem. Clean the list; your domain is fine.

What 553 5.1.3 means

Gmail553 5.1.3 The recipient address <address> is not a valid RFC 5321 address.

The address breaks the rules for what an email address can look like.

Any server5.1.3 Bad destination mailbox address syntax

The standard meaning: the address is badly formed.

List problem or sender problem?

List problem. The address is malformed, so no server could deliver to it. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 553 5.1.3

  1. Look at the address in the bounce: spaces, commas, two @ signs or a trailing dot are the usual culprits.
  2. Correct it if you can tell what was meant; otherwise remove it.
  3. Spot-check rows after every list import, so broken addresses never get a send.

In SendingOS, a 553 5.1.3 bounce shows its reason and code on the conversation, and the thread is labelled Bounced, so you can filter the inbox to bounces and pull these addresses before they add up.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

550 5.1.1

Sources: Google: Gmail SMTP errors and codes · RFC 3463 · RFC 5321