A customer called, said that when their Mac is turned on (OS X), other PCs on the network cannot access mail and the Internet gets slow. I noticed that there was some delay on the Mac sending e-mails with attachments. Earlier in the week, the router an modem were replaced and the problem seemed to go away with that, however I'm told the problem is back and they're noticing it when the Mac is on, and that the problem goes away when it's off.
I'm not sure if that sounds far-fetched, but do you have any thoughts on it? The Mac is using Mail for the e-mail, and a built-in wireless card for the Internet.