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Email rejected due to failed authentication checks  

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User has an @cox.net address.  Suddenly having trouble sending to a certain long standing correspondent.  When she hits "Sent" is processes it for a few second and sends it to the Outbox.  When I look at it a little deeper, there is an error and the SMTP server points us to a webpage of potential errors.  Tracking this down, the error is "your email failed authentication checks against your sending domain's SFP, DomainKeys or DKIM policy."   I vaguely know that this is a system of verifying email addresses.

My question is, when you have a domain such as @cox.net that you don't manage, how do you get around these sorts of authentication checks on the recipient's side?

Interestingly, when my friend sends a brand new email to this same person, it arrives.  The rejection ONLY happens when she replies to one of his emails. I've checked the "reply to" address in the header of a received email, and it's exactly the same as the emails that are successful.

Look a bit deeper at exactly which SMTP server is being used in each case and with what credentials.

Note that if the mail fails to send (ie. lands back in the Outbox) then it is the *sending* server that is rejecting the attempt, not the recipient’s server.

Are there any alias email addresses set up such that the reply is actually trying to send from an alias?

1 Answer
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First I would check if the IMAP settings are correct, https://www.cox.com/residential/support/cox-email-server-settings.html

I would also try their webmail, if they have one to see if it works there.

The other thing I would try is create a new user account on the Mac and setup the cox email account there and test.

It's possible that an item in the keychain is faulty?

Have you also tried connection doctor in Mail.app?

I know that I am kind of throwing my towel around but perhaps these first few steps might help to understand the problem without even knowing what those errors precisely mean in detail. You can also check https://www.cox.com/business/support/changing-email-server-settings-for-mail-on-macos.html

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