That's because the poster probably thought that `tr` works like `sed`, replacing the string "spam" with the string "du" to produce their email address, du@50km.com.

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.