can't receive e-mails (from Upwork.com)

can't receive e-mails (from Upwork.com)

Hello. My Zoho e-mail addresses won't receive any e-mails from Upwork.com. (And no, they're not in my spam folder.) Any idea what's going on? Is Zoho blacklisted, even from receiving e-mails, from some servers? Or is there some other issue? 

This is for an e-mail account with my own custom domain (not Zoho.com).

Any advice would be much appreciated. (Wondering if I need to move to another hosting provider.)

/David