A specific sender can't send mails to my domain

Hello there, I have a problem with my client. He send emails using outlook (user@domain.com) to me. But the bounce back mail is shown to him. [quote] Your message wasn't delivered. Despite repeated attempts to deliver your message, a connection to the remote server was closed abruptly. Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that your email system is unable to connecto their email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem. For Email Admins This often indicates the recipient's firewall SMTP fixup setting or other firewall settings are preventing the SMTP protocol negotiation from succeeding. For more information and tips to fix this issue see this article: