I have set up (or tried) a new user to our HubSpot account. I also have access to the email server, so I've followed the advice of whitelisting you already.
I have already set up other accounts, mine included in the same domain without issue, however, on this occasion, I am not getting the invitation emails for this user.
chet@exilier.com
Can you see if you can spot anything your side or push the message through manually?