Sometimes nothing happens when printing to network printers

I've been having trouble printing to network printers from KDE and Gnome applications lately, using CUPS or LP. No errors show up, and as far as the applications are concerned, the documents printed. But the documents are never actually sent to the printers. There's no problem with the printers

How to allow outgoing mail to certain domains with exim4?

I would like to configure exim4 to allow outgoing mail to certain domains. I've been unable to figure out how to do so from the manual or using the dpkg-reconfigure utility: messages are bounced back with the error that mail to remote domains is not supported. I use a laptop and regularly switch b