The best reason I can come up with for a disposable and temporary email address is so that you can sign up on a website without having to worry about all the spam email they are going to send you. And don’t forget all the other spam after they sell your address to other companies/spammers. (I have so many products to make me bigger, that I need new pants.) Some other benefits of temporary email are…

No sign up and/or registration
Automatic expiration date i.e. (10 minutes – 3 months)
Message forwarding to your real email address

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