Please ensure that the Exchange SMTP receive connector allows anonymous sending of emails, this is important for incoming emails. And please turn off SMTP authentication in the SmartPop2Exchange SMTP configuration if the problem persists and it is turned on.
Your Exchange/SMTP server is not configured to receive mail for yourdomain.com. From your Windows Start Menu, select Start | Programs | Microsoft Exchange | System Manager. There select: Recipients | Recipient Policies | Properties. Click on the "email Addresses" tab, under Generation rules, select SMTP, and then click Edit and change the value to "@yourdomain.com". Please also make sure that the email addresses of all users are specified in their Active Directory entries on the "email Addresses" tab.
"unable to relay..." means your SMTP server does not allow SmartPOP2Exchange to send mail to it (as a SMTP relay server). Please first check the entered mail addresses in the "SMTP settings" of SmartPOP2Exchange. Make sure that these addresses also exist on your SMTP server. If you like your SMTP Server to send/forward that mail to an external server then set your SMTP server to allow all SMTP relays or at least relaying from SmartPOP2Exchange. (For MS Exchange Server you will find this setting in the properties of the "virtual SMTP server")
This notification is typical for Exchange Server 2007/2010 because they both have a standard option limiting email sizes to 10MB. These can be modified by changing the limitations of the Receive Connector. This occurs over the Powershell: Help article at Microsoft (Set-ReceiveConnector ---> MaxMessageSize)
Enter the following lines in the command line to simulate sending an email via SMTP to the Exchange Server:
telnet [Your Exchange server's IP] 25
MAIL FROM: firstname.lastname@example.org
RCPT TO: [your email address]