A "Mailbox full" error message indicates that the email account which weighsoft is liked to is full and cannot process any more incoming out outgoing emails.
This email address will be on your own email server (due to the fact that Weighsoft does not send out emails directly but rather passes them to your own email server to be sent).
You can see what mailbox Weighsoft is linked to by going to Administration>Settings>Email or Administration>Lookups>Company>Emails.
Once you has located the mailbox you will need to talk to your IT for them to clear the mailbox out and allow traffic to recommence.