Email to Zoho gets bounceback error: 550 5.1.1 <....@....com> "User unknown"

Email to Zoho gets bounceback error: 550 5.1.1 <....@....com> "User unknown"

Here is the content of the bounceback:

.... [Gmail headers] ...

Delivery to the following recipient failed permanently:

[my user]@[my domain]

Google tried to deliver your message, but it was rejected by the server for the recipient domain [my domain].com by mx.zohomail.com. [74.201.154.201].

The error that the other server returned was:
550 5.1.1 <[my user]@[my domain].com>... User unknown

---- Original message ----
...[additional Gmail headers and mail content]...


Any feedback is appreciated.