We have an IIS 6 SMTP server that is relaying messages from applications and devices to our users on an Office 365 tenant. Recently, users have not been receiving certain messages that were generated by our ERM system using the smtp relay. The procedures on the ERM server logs report as completed successfully but the smtp server logs do not seem to show these entries. There are a number of messages that do go through and they do show up in the SMTP log files. Are there restrictions in the SMTP virtual server that would drop the connection but not show up in the logs? Also on the Office 365 end, are there any logs that would show a failed connection from this connector? The connector is set up with the conditions that is must come from the specified IP and the sender email address is from an accepted domain. Any help would be appreciated.