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Mailbox unavailable. The server response was: 5.7.1 Message rejected as spam by Content Filtering.

Hi I have a unmanaged dedicated server in a data center on that I have a few website but one of them I have just set up a new site using nop commerce and works great but when it sends mail to my internal server I get an error message in the logs saying "Mailbox unavailable. The server response was: 5.7.1 Message rejected as spam by Content Filtering." I don't know where to look, is this an issue with the dedicated server or my internal one with exchange? the other thing I don't get is if I send a test message form nopcommerce it works fine, plus all my other sites are ones I have wrote myself and the emails go to the same server in the same way and works fine but are different email address. I read some where to just run a command "Set-ContentFilterConfig -BypassedSenderDomains yourdomain" but which server and in what program as does not look like a cmd.exe command. Please help.
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I recommend you whitelist the IP of your commerce server. To do this in EMC, go to Server configuration -> Hub transport. For each of your HT servers, Click on the anti-spam tab. Make sure IP Allow list is enabled. Open the Properties page for the IP allow list then add the IP on the allowed addresses tab.

What email server are using?  Exchange?  Built-in SMTP server?

Have you tried changing the nopCommerce SMTP settings to include a username and password?
FYI, the "Set-ContentFilterConfig -BypassedSenderDomains yourdomain" command can be run from the Exchange Management Shell (aka Powershell for Exchange).

You can configure Exchange to bypass spam/content filtering for that particular user as well.

You should not whitelist domains or even email address as these can easily be spoofed. Best practice is to whitelist IP addresses.

Before you "white list" anything make sure it's not the content of the email.  If the content is just test content (especially if it says "test") or has very little content at all, the content filter will stop rejecting it once you get enough "good" content in your email.

FYI, you can also white list it based on the actual content to the email to: