Here's how. Each message must have a valid from address. This address is
validated against and ISP's database. If the account exists, then the
mail
is allowed to be received, otherwise it is thrown in the trash. It's very
much like putting a return address on an envelope, but since
computers/networks are so fast we can verify in real time that it is
fast.
Any mail without a return address gets put in the trash (and this is
Sadly, the network isn't guaranteed to be either fast or reliable.
A system like this has to cope with "TEMPFAIL" type situations.
Also this kind of technique imposes a cost on victims of forged from
addresses. This has been covered before (so why am I saying it again?)
While this isn't a fatal objection - it doesn't seem at all fair.
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