Error "553 5.1.7 sorry, mailbox syntax not allowed"

Products

• Email Security.cloud

Issue/Introduction

The recipient's mail server responded with "553 5.1.7 sorry, mailbox syntax not allowed," indicating that the email was rejected because the email address contains an invalid or disallowed syntax.

 

Environment

Email Security.cloud 

Resolution

Check message To: and From: headers and ensure that the email addresses are correctly formatted.