If not, eventually someone will do something really shitty to you. And you will feel crappy and angry, but you will try to be above it. And move on. And only envision their comeuppance in cartoony, surreal daydreams. Superior, but detached, superior, but detached, superior but-
THEN one day you will be shuffling through your e-mail and just happen to find some very incriminating e-mails written by said person. Nothing really horrible, just relationship-ruining and potentially life upheaving.
And you know you'll never actually do anything with them. But now you have to look at them. And ponder. And think very evil thoughts about what you could do, were you not the specimen of grace and maturity that you are. Thoughts that don't involved brightly colored anvils and horrible haircuts in the shape of ten foot cactuses. Thoughts that involve clicking send and seeing the repercussions unfold in a very real, kitchen-sink-y kind of way.
And you'll wish you had deleted them way back when you got them. Because then you could move on with your life, feeling classy and self-important, instead of starring at 3 month old e-mails with steam coming out of your ears, feeling like a shitty person for even thinking such bad thoughts. Especially if you're not Catholic, and you'll have to deal with the guilt from your mean imagination all on your own. Or with Ben and Jerry. (Which is problematic, especially if you're not bulimic. And yes, bulimics and Catholics are obviously shame free.)
So just do it. Delete your entire inbox. Free yourself from the potential bad thoughts and get down with your Zen self. Unless you want to keep them saved. Like- you keep them with the express purpose of harnessing their potential destructive power in the future. In which case, shame on you!