Third-Party domain email issue

I am working with a client who owns a domain from Greengeeks. By updating the DNS settings, I was able to successfully connect the domain to their new Squarespace website. So that part is working. When I try to test their email through Greengeeks, I can send emails to anyone, but I can only receive emails from one of the email accounts on their custom domain. So, no external emails have been received.  I also followed the Squarespace guides on connecting the domain, and they say I do not need to add any mx records.  Does it take up to 72 hours for email to work? I thought since email was handled through GreenGeeks, this was separate from the connection to Squarespace. Any thoughts and help are appreciated.

The email should be separate from Squarespace, and any if required mx records are added to the Domain DNS settings. I don't think it should take 72 hours to start working, it does not with Google Workspace.