That's a preposterous interpretation. 18 USC 2511(2)(a)(i) says:
It shall not be unlawful under this chapter for an operator
of a switchboard, or an officer, employee, or agent of a
provider of wire or electronic communication service, whose
facilities are used in the transmission of a wire or
electronic communication, to intercept, disclose, or use
that communication in the normal course of his employment
while engaged in any activity which is a necessary incident
to the rendition of his service or to the protection of
the rights or property of the provider of that service,
except that a provider of wire communication service to
the public shall not utilize service observing or random
monitoring except for mechanical or service quality control
checks.
See http://www4.law.cornell.edu/uscode/18/2511.html. In other words,
reception of the mail is legal, since that's a necessary part of
the service delivery, and given how much load spam places on ISPs
rejecting it is permissible as protecting the property of the service
provider. As for the last clause -- the law defines "provider of
wire communication service" as (more or less) a phone company; ISPs
are "electronic communication service" providers, and hence are not
covered by that.
Since I get less spam in a normal day than I do regular mail, and I get
a lot more spam (and a lot more regular mail) than the average person,
I wouldn't want to try to fight from that position in court.
"But your honor, 5% of his email was spam, we had to kill that in order
to provide the service."
"But you were killing all his legitimate mail traffic from the 2nd-largest
ISP in the country."
"We had to, he got three spams from them last week."
Rings kinda hollow.
It gets worse if you killed what you thought was spam, and it was
SOLICITED commercial email he signed up for. I get quite a bit of that.
That's a preposterous interpretation. 18 USC 2511(2)(a)(i) says:
It shall not be unlawful under this chapter for an operator
of a switchboard, or an officer, employee, or agent of a
provider of wire or electronic communication service, whose
facilities are used in the transmission of a wire or
electronic communication, to intercept, disclose, or use
that communication in the normal course of his employment
while engaged in any activity which is a necessary incident
to the rendition of his service or to the protection of
the rights or property of the provider of that service,
except that a provider of wire communication service to
the public shall not utilize service observing or random
monitoring except for mechanical or service quality control
checks.
You may have a very hard time convincing anyone that rejecting significant
amounts of legitimate mail is necessary. Necessity is a requirement for
exemption under this section.
Note that service observing and random monitoring are permitted only for
quality control checks. If headers are analyzed to slap up automatic filters
when suspicious patterns are detected, it may be necessary to defend this
practice as a "mechanical or service quality control check". This may be
more difficult than you would expect.
If automatic spam filtering is considered to be "service observing or
random monitoring" than those who employ it may well face legal action under
this section. Notice that the exception for protection of property doesn't
apply to this category of interception.