Seriously.
How can you tell if it’s a “bad link”? Use the mouse-over pop-ups to see where it really and truly goes, and if it’s not where it should be going, don’t go there.
Apparently the “undeliverable” virus continues, but is now claiming to come from the USPS…. don’t click on the attachment, nor any links the email contains. Snopes: Undeliverable Virus
If you mouse-over a link, the ACTUAL destination should show in the left hand side of your status bar (on the bottom of your browser) — if it doesn’t match what you expect — do not click on it. Your email should show a pop-up of the actual destination for any links in emails as well.
The domain name for USPS URLs should end in usps.com — meaning it should look like this http://blahblahblah.usps.com/whatever
If the last thing before the 3rd / isn’t the domain name you would expect to see — then it’s not going to that site.
A fake URL might look something like this http://usps.blahblahblah.com/whatever
unless the usps.com is the very last thing before that 3rd /, then it’s not going to direct you to usps.com. It’s going to direct you to whatever domain/site the blahblahblah.com is.
I did a little image to show what I’m talking about…….

In this instance the link goes to yahoo.com — if it were aiming at someplace unexpected, or untrusted — just don’t click on it.
Also, in the case of email — check the headers to make sure the source is what you expect.
Here’s a gmail header — you can access this by selecting the down arrow next to the [Reply] button and selecting “Show Original” — other email systems have other mouse-clicks to get to the same information,.

If the domain the email is coming from (see the red boxes) doesn’t match the From: address — chances are that’s not where it came from — don’t click on the links, or open any attachments.
I hope this helps you stay safe on the web.