Other than the NSA, I have not heard of significant cases of traffic intercept since Stoll’s Cuckoo’s Egg (where he hooked line printers to serial cables).
Nowadays with extreme high speed serial fiber and T3/OC3 you would need some serious technical equipment to intercept transmissions. and even more serious equipment to record and analyze all that traffic. Even compromising routers at one end instead of physical intercept, unless you have a specific target in mind, you will generate far too much traffic to route a copy to yourself. That’s why the NSA installed their own hardware in the telephone company closets to spy on Americans in contravention of the laws.
As for local intercepts - again, unless you compromise the local switch system (a bit easier with physical access to the switch) and make your personal line a monitor port - it’s not possible. Original ethernet braodcast all traffic to everyone on the LAN, but modern tech uses switching - a packet only goes to the wire that the recipient is on.
the more likely candidate is Starbucks. If you are sitting in a coffee shop using wireless, all that traffic can be read; the time to break a wifi encryption, especially the older ones (WEP), is minimal. But then, your computer is exposed to anyone else on the same wifi - hopefully you have secured it so that “EVERYONE” does not have read, or worse, write, to your disk; and you’ve applied WIndows Updates so the known security holes are plugged. Also, if your IT people are clever, you use VPN from there to the work network, or HTTPS to your web email, so there’s an added layer of traffic encryption much more difficult to break.
I suspect most email compromise happens at the ends - people who compromise a server and gain (mail) administrator access would have free reign to read all emails on that server. People who crack your email password can read your mail.
In fact, Stoll’s book is a fascinating read to see how lax security was in those days. Hackers who gained root (admin) access simply scanned the disks for “password”, for exampl. They would find memos (“new admin password is XXXXX”) and emails (“I’ll be gone for a month, here’s my user and password if you need my files…”).
Nowadays, people keep old emails around for years. (I just checked, and I have useless junk as far back as 2002 despite changing computers about 5 times). Someone doing a search will breach your process - the first email will have the word “password” and “I will email you the…” so the perp only has to look for the next few emails from you; odds are the one that has only one word - “Plugh!123” is the one with the password.
So the short answer is - don’t worry about interception. Worry about end computer compromise.