I am running Groupwise 2012

One of my users gets this error when opening their calendar in Webaccess

(D101) User not found on post office

In the webaccess log this is the error message

8:50:19, <SOAP>, -, INFO, "user" , Check Calendar
8:50:19, <SOAP>, -, INFO, "user" , Item count - 38
8:50:19, <SOAP>, -, INFO, "user" , [53505] User not on post office

Is there anyone who knows how to resolve this issue