This doesn't make any sense to my terminal. I think your domain is supposed to be 50km, but my terminal thinks its ku.cou.
That's not what `tr` does. It replaces each letter S-P-A-M with the letter in the same position in the string D-U. Because "du" is shorter than "spam," `tr` just reuses U for letters 3 and 4. So the mapping is S->D, P->U, A->U, M->U, making "spam@50km.com" into "duuu@50ku.cou," and the -s flag then compresses multiple repeated U's into one.
Lesson: don't be too clever.
A few other lessons from this: 1) test your scripts, 2) be clever enough, 3) email obfuscation stopped working in 2000 for preventing spam, 4) handle spam at your receipt