I'm looking for a way to intercept outgoing SMTP messages from a few users on my network. Sadly I'm dependent on an e-mail provider and can't set up my own e-mail server.
You need a certain amount of control over the network in order to intercept any traffic; what sort of control and what sort of hardware can make your job harder or easier.
A managed (even lightly managed, like a Netgear FS726T) switch with a monitor port will get you most of the way there, and Wireshark plugged into the monitor port watching for SMTP traffic from the needed IP addresses will get you the rest of the way.
Without a hardware monitoring solution, or access to one, you'll need to work on it with a more ...malware-esque solution on the individual computers, which I won't go into yet, but maybe post a little more about what tools you have available: What type of router are you using? Some of those will echo traffic. What type of switch? Is there a spare box around you can throw Wireshark on?
Firstly - are you legally allowed to and even if you are most countries insist you let your staff know that this is going to happen, there's usually a period of time that has to go by between telling them and starting monitoring.
Secondly - the easiest way is to create your own MTA internally that allows for mail-snooping that just forwards on smtp to the usual provider. If you can't do this then wireshark will let you do that kind of thing but is a lot harder to do.
Are you sure you want to do this?