Just what the title states. A colleague mentioned that it may be possible for a router/gateway to be configured to cache everything that passes it's way (mail, attachments ...)
Say a mail with an attachment is dispatched from one company to another on a different continent; could somebody performing housekeeping at the ISP view the mail, and linked attachment?
This question is a bit open ended. A Router by its strictest definition simply routes packets at Layer 3.
So, essentially, no. I don't know of any normal off the shelf pure router that have this sort of behaviour built in.
However, there are most certainly other devices that could. Now, the confusing thing is that these could be bundled with Routing technology and may be even be badged as such.
So, if the argument is "IP traffic is insecure by default" then your colleague is absolutely right. However, it's unlikely be simply house keeping - just think how big this cache would have to be.
Routing is just one part of getting Internet traffic from A - B and you can bet your e-mail goes through all sorts of other devices inbetween.
The role of router as you find it on the networking diagrams doesn't do that sort of higher-layer caching. It just forwards packets to their next local network, rewriting the layer two data as needed.
Where it gets confusing is when the routing function is bundled into something else like a firewall. These days "firewall" can include everything from stateless port block/allow statements at the most basic, to application-level content filtering (smart enough to notice that someone is trying to mail company secrets to a competitor). The most featured of these devices aren't called Firewalls, they're called "Security Gateways" (at least this year) and 'firewall' is but one feature on the list.
These Gateways can very well cache content passing through them. Their whole point is to automate content inspection to a very detailed level to prevent key data going missing, and preventing users from accessing bad content.
Security Gateways, which have 'router' among their long feature lists, are typically deployed at the organizational edge. Because of how they work, they need to be placed in the network path of the interesting traffic, which is why they're typically the first or second device on the network after the ISP network-cable.
Where these gateways are NOT deployed are on ISP internal routing networks. Some ISPs offer these features as an extra service you pay for, so you'll know if it is happening. For ISPs, all they care about is Layer 3 transit unless you have a separate agreement in place where they care about higher level stuff too. Doing 'routine maintenance' does not expose reassembled client data to their technicians; if they're doing packet traces to isolate a problem client data may be in the capture, but they would have to go through significant, purposeful effort to turn that data into files/email.
It's not likely to cache it on the router itself, but it's certainly possible to configure another port to mirror an active port, and attach a device to it that records all the packets received. You should always assume that ISP staff can view any unencrypted traffic that passes through their ISP.
If you are talking about IP-level routers, this is highly unlikely. Routers are responsible for handling huge amount of traffic especially in busy/big ISPs. So, it is really inefficient to do such things. This is not the routers task to do so. Usually, they don't (need to) understand application level headers and data.
