https://www.techieshelp.com/exchange-2013-enable-anti-spam/

https://www.informaticar.net/exchange-server-whitelisting-domains-and-email-addresses/

https://blog.ahasayen.com/smtp-relay-exchange-internalsmtpservers/

https://docs.microsoft.com/en-us/Exchange/antispam-and-antimalware/antispam-protection/antispam-on-mailbox-servers?view=exchserver-2019

Get the current list:

Get-TransportConfig | fl InternalSMTPServers

Replace list:

Set-TransportConfig -InternalSMTPServers 192.168.1.0/24, 192.168.5.2

Add to list:

Set-TransportConfig -InternalSMTPServers @{Add="<ip address1>","<ip address2>"...}