These are common types of problems you might encounter with the SMTP-proxy, and the solutions that most often resolve these problems.
For information on how to make sure your IP address does get blacklisted because of spam or from being an open relay, see the knowledge base article Protect your External IP Address from SMTP Blacklisting.
In some cases, email attachments sent through the SMTP-proxy appear to the recipient as Winmail.dat attachments to the message. For information on this issue, see the knowledge base article Why do email attachments through the SMTP-proxy appear as Winmail.dat attachments.
For more information on errors that occur when you use TLS mail delivery encryption, see the knowledge base article Unexpected delivery errors from TLS Encryption with the SMTP-proxy.
In some cases, this error can occur when the SMTP-Proxy maximum header size does not match the Exchange server header size. For information on how to set the SMTP-Proxy maximum header size, see SMTP-Proxy: General Settings.