Quoting "Andreas S." <f / andreas-s.net>:

> It is trivial to create a formally correct address that makes
> absolutely no sense, so what's the point of doing such a
> complicated and error-prone validation?

Well, I might actually have one.

The comment form on my web site sends email directly to me; as a
convenience, the email address entered on the form becomes the
email's From address (I can see who it's from and reply more
easily).

Now, doing that would open up all sorts of injection attacks if I
didn't do any validation.  So I do a quick and paranoid (syntactic)
validity check -- if the address fails, then it is included in the
body of the message instead of a header field.

In this case, a nonsensical address is perfectly fine (I will see it
and know better), and it's even okay if a valid address is rejected
(I'll still get the message and be able to figure things out from
the body), but I have to be able to detect syntactically invalid
addresses.

-mental