The email used on outlook or from accessing it from a website works fine

However when using Mac Mail, the error message "cannot send message using the server" appears when trying to send to a single email address (another email using the email array service). Sending to anyone else sends it find.

We thought there was something wrong with the outgoing server name (in this case and tried default, different recommended ports and the same ports used for Outlook.

What would be the first place to start to narrow down a solution as to why Mac Mail cannot email to this one email address, but can use it on outlook or from the website ? (and we did verify spelling of the email).