Our corporate email is hosted on Google Apps, along with 5 domains for various products. Internally (Behind our firewall, not exposed to the internet) we have an IIS server running Microsoft ESMTP MAIL Service, Version: 7.0.6002.18264
This SMTP server is only used for internal applications to message users in the organisation, backup alerts, scheduled scripts notifying admin accounts, etc...
It has worked for many years, but as of last week, stopped working, the message I am getting is FROM google, when my SMTP server attempts to deliver to the end user, I am getting
Final-Recipient: rfc822;email@example.com Action: failed Status: 5.5.0 Diagnostic-Code: smtp;554 Relay Access Denied - psmtp
So my SMTP server is accepting it, trying to send it to google who hosts the mail account, and it stops there, with google's mail server saying the relay is denied.
This makes my SMTP server send ALL mail to the badmail folder.
The messages contain no spam content, and in fact one of the processes is the Google Apps Sync for Active Directory integration reports, so a google product configured like their integrator left it.
So I can only assume google implemented some new criteria on what is and is not acceptable for other SMTP servers to be allowed to send mail to them, but I cannot put my finger on what.
Nothing in our configuration on most of this has changed in years, and it has sent hundreds of messages daily during that time.