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We have an issue with single company trying to email us. The failure code is #4.4.1 which means a connection time-out. However, to my knowledge no one else that is emailing us is having this issue.

They sent me the bounce-back and the generating server was their email server. However, one line of it I'm unsure about:
Final-recipient: RFC822; user@ourdomain.com Action: delayed Status: 5.4.0.
Maybe they typo'd the email address or something? I read that 5.4.0 means the address was not found on our email server. 

Is this issue on our side or theirs? What (if anything) can I do to help fix this? We have on-premise Exchange 2007 (in an SBS 2008 VM).

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If the email address wasn't valid, then the email would bounce back immediately, either from their server (when you are using recipient filtering) or after delivery by your server. Therefore I don't think it is a typo.

As the recipient, there is very little you can do to troubleshoot. Until the email is handed to your server it isn't under your control.

The sender needs to do the troubleshooting, including attempting a telnet test to your server etc. It could be a routing issue for example. If they are a local company they could be using the same ISP and the routing problem is there.

The NDR on its own is almost completely useless.

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There was a Geo-IP firewall rule (country block) that was causing their email not to get to us. I disabled this and all was well again.

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