It is an option on one of the configuration panels within the program.
Clueful people have found that and turned it off. More-clueful people
have turned off Inbound and Outbound email scanning, as it isn't
necessary. (But I guess it gives warm, fuzzy feelings to newbies.)
That "scanned" message is merely-an advertisement, 'cause there ain't no
way it can guarantee what the message says.