I have a supplier that cannot send email to me. When they send an email message they receive an undeliverable email message that says:There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.<My_Supplier's_Domain.com #5.5.0 smtp;503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.>I'm using an Exchange 2007 server running on Windows Server 2008. I receive email from other people at other companies. Is this a problem with my Exchange 2007 server settings or is it a problem with my supplier's mail server? What can be done to resolve this problem?

Hi,Is the supplier used your Exchange server to relay the email when sending the external email?If it is, please try the below article:http://msexchangeteam.com/archive/2006/12/28/432013.aspxIf not, this is the issue of the supplier's mailbox server. Please ensure the supplier's mailbox server has the permission to allow themself send the outbound email.ThanksAllen