need help ASAP, error to deliver virtual addresses

[Iwao Makino]

hello all,

I have problem reported from my clients that they can not receive
e-mail under our virtual domains.

and when i look at /var/log/maillog, the following error shows up

qmail: 921181110.478964 delivery 2387: deferral:
Unable_to_forward_message:_qq_write_error_or_disk_full_(#4.3.0)./

and I know the disk is not full...

can anyone suggest how to fix this problem?
it's working for my own domain's virtual addresses.

tia,