Error recipient ok

Question / Problem

"Sender ok" / "recipient ok" error when SmartPOP2Exchange delivers e-mail to the Exchange Server.

Answer / Solution

This "error message" is caused by greylisting solutions.
For example, if you are using ESET for Server, even if you disable anti-spam capabilities in its configuration, it will not be greylisted.
To disable greylisting, the Exchange Server Transport Agent must be disabled by ESET: (in the Exchange Server PowerShell)
Disable-TransportAgent -Indentity "ESET FilterAgent