Is this a hard bounce?

Yes, this is a permanent failure. The email will never be delivered until the address syntax is fixed.

Can I automate the fix?

You can use regular expressions or email validation APIs to clean your list of malformed addresses before sending.

permanent Mailbox

The recipient's email address is formatted incorrectly and violates SMTP standards, so the server rejected it.

The email address contains invalid characters (e.g., spaces, multiple @ symbols).
The address is missing the domain part (e.g., 'user@' instead of 'user@domain.com').
The application failed to properly enclose the address in angle brackets (<>) during the SMTP transaction.

Validate the recipient's email address and correct any obvious typos.
Implement strict email validation on your signup forms to prevent bad addresses from entering your database.
Ensure your sending script properly formats the RCPT TO command.

Amazon SES. Often seen as 'RCPT TO syntax error' if the address is not enclosed in '<>'.

501 5.1.3 Bad destination mailbox address syntax.

Is this a hard bounce?: Yes, this is a permanent failure. The email will never be delivered until the address syntax is fixed.
Can I automate the fix?: You can use regular expressions or email validation APIs to clean your list of malformed addresses before sending.