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If you send an email using the same FROM address, but you don't use the ISP's outgoing mail server, this cannot happen. As a result, anyone that receives that email (e.g. another ISP) cannot verify that the message was authorized.
This can result in serious deliverability issuesis a potential deliverability issue.
We strongly recommend that you switch to using a FROM email address associated with a domain that you control (e.g. your company's domain name).