Yeah, the Matrix rules are kinda vague. Far as I know, it isn't explicitly stated that you can hack the 'Ware of a dead Sammie. Makes sense, though, so long as the wireless is on. Scared the hell out of my party last session, when the dead elven Sammie in the women's washroom dropped their Bio-ed soldier with a kick between the legs.
Most of the time, you need to apply a level of rationality to things. IRL, you can hunt an email back to the server that hosted the sending account. From there, they have the IP address of the sender. Which means, in SR terms, Email leads back to MSP, which should have a record of the sender's Commcode. If it was forwarded from another account, then you get bounced around from x to y to z to x to a to z, which is where it starts chewing up your time, but if you can keep your head on straight, and have the time to go through all the records, after hacking into so many databases, you'll eventually find the Commcode of their disposable Link. You can then, through complicated programs, figure out where they are, based on what nodes they're pinging to reach the other side of the world.
Sending messages is pretty damn safe, if you've got the right safety procedures in place.
The Matrix does relate to our Internet, but it's on a whole new scale of complexity. It's totally possible to comprehend it, but a headache to try.
Ruling-wise, personally, I would have thresholds based on, primarily, how many servers the message was bounced off. (+1 per 3) If the sender uses a disposable Link, then it's a race against time - can you triangulate the rough area they sent the message from, before they're well away from it, blending with the crowd?
High-residence areas will be considerably harder to cope with, but easier to reach, while secluded areas will have less nodes to narrow down from, and less people to confuse, but it'll take longer to get there.