Aparentemente, isso é um bug no cPanel 11.38. Eu abri um ticket de suporte com o cPanel e a resposta deles foi:
A new internal case, 68457, was opened by Nick concerning the issue with the queued domain handling for users outside the jail or virtfs environment. A new router is set to be added for handling in that case, which would deliver the message via SMTP rather than using the prior /etc/queuedomains behavior.
When this case has gone into production, it would appear in http://go.cpanel.net/changelog for 11.38 tier. You can see which version is available for each tier at http://httpupdate.cpanel.net location.
A atualização para o cPanel / WHM 11.38.0 (build 10) resolveu este problema.