I am using a smarthost for sending mail from my XEAMS server.  It is in a single standalone setup.  the smarthost configuration hadn't changed and I have verified that the upstream relay is working properly as designed.  My Xeams server does not send mail now and mail is backed in the queue.  Whenever I try to make a manual send from the cue I get the following error.

Error: Unable to connect to remote SMTP server. 501 5.5.4 Invalid Address

All setting in the smarthost are correct.  I have even changed it to point to a local relay that requires no authentication and the error still happens.  I have tried deleting the smarthost config file and let it recreate a new one with the setting. I have remove all settings rebooted the server and put them in, and the same issue happens.  Bottom line is the server is not sending the mail to the upstream server.      I am running Version 7.1, build 6090  I am guessing this happened around early december not sure the exact date as I don't send much mail. Based on the last sucesfule email i sent to an external account, it was probably after the 4th. 

Any thoughts?  

Any help would be appreciated.

Couple of suggestions:

1. Since you do need authentication in order to send messages to the downstream server, remove the smart host. Instead, add ALLDOMAINS in the local host file. Here are the steps:
   • Click Manage Local Host File under Server Configuration
   • Add a new domain as ALLDOMAIN referring to the IP address of your downstream server
   • The benefit of using ALLDOMAIN vs smart host is that SMTP communication will get logged in SmtpOutboundConversation.log and it will be easier to troubleshoot
   • Note that messages already stuck in the queue will still try to go through the settings in the smart host.
2. Use EmailSender to manually send an email to your download stream server and look at the SMTP log to see why is it not getting delivered.