Can send email only to certain addresses

Hi, Recently we implemented an email account to our WordPress domain we are using, unfortunately what we've found is that email is considerably slow, and it refuses to send to certain addresses that we know exist, and then at other times it will send just fine. Below is redacted copy of whats sent through when it fails to send: ---------- Forwarded message ---------- From: Mail Delivery System Date: 16 August 2018 at 08:03 Subject: Mail delivery failed: returning message to sender To: [redacted] This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: [redacted] No Such User Here Action: failed Final-Recipient: rfc822;[redacted] Status: 5.0.0 Is there something we are missing? The email address is linked to another gmail account and is accessed via gmail, with the settings implemented through gmail so that both addresses receive in the same inbox. Cheers!