Got it -- so yes, this ties back into Bryan's comment -- you're using the Person-level "Proxyemail" field address as the From Address/Reply-To in your alert, but assuming that Proxy Email is the value that is written by the person in the form, there's no way to ensure that it's going to match the signing domain that your Marketo instance is sending email from, and it's going to in all likelihood fail SPF/DKIM validation . * Your company's receiving email server is basically interpreting the email as saying it's coming from a person at one domain, but actually being signed by your Marketo sending domain, so it's flagging it as potential spam. Like Bryan mentioned, changing the From and Reply-To fields to be addresses at your internal domain should resolve this. * *Edit: See Sandy's clarification below re: the technical aspects that are likely at play here.
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