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. DS