I want to refuse unauthorized email that has a from address @mydomain.com. The only way I've come up with to do this is to have a sender blacklist of *@mydomain.com. This has worked well until now because I need to receive email from Zendesk and the from address needs to be @mydomain.com. This normally wouldn't be a problem; I would add the additional SMTP server to the Host-based relay whitelist. But in this case Zendesk uses Gmail to send email and I guessing quite a few spammers do too. Is there a better or more elegant way to solve this than whitelisting all the Gmail servers to open relay?
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