553 5.1.3 Invalid recipient address syntax
Meaning
The recipient's email address is formatted incorrectly and violates SMTP standards.
Common causes
- The email address contains invalid characters or spaces.
- The address is missing the @ symbol or domain.
How to fix
- Validate the recipient's email address and correct typos.
- Remove the malformed address from your list.
Provider notes
Gmail. States 'The recipient address is not a valid RFC 5321 address.'
Example bounce
553 5.1.3 The recipient address is not a valid RFC 5321 address.
FAQ
- Why did my application accept this address?
- Your application likely lacks strict frontend email validation, allowing users to submit malformed strings.