17. A computing system comprising: one or more hardware processors configured to execute instructions; a tangible computer readable medium having modules including instructions stored thereon, wherein the tangible computer readable medium is configured for access by the one or more hardware processors to execute the modules, the modules including at least: a first module configured to access one or more electronic transmissions transmitted from a client computing device or ready for transmission from the client computing device; a second module configured to, for respective electronic transmissions of the one or more electronic transmissions: scan content of the respective electronic transmission to identify one or more keywords included in an advertising data structure; select one or more advertisements for insertion into the respective electronic transmission based on relationships stored in the advertising data structure, wherein the stored relationships indicate associations between the identified one or more keywords in the respective electronic transmission and the one or more advertisements; insert the selected one or more advertisements into the respective electronic transmission, wherein inserting the selected one or more advertisements comprises changing at least one display characteristic of one or more words already included in a message body or subject line of the respective electronic transmission; and initiate transmission of the respective electronic transmission to one or more intended recipients.
17. A computing system comprising: one or more hardware processors configured to execute instructions; a tangible computer readable medium having modules including instructions stored thereon, wherein the tangible computer readable medium is configured for access by the one or more hardware processors to execute the modules, the modules including at least: a first module configured to access one or more electronic transmissions transmitted from a client computing device or ready for transmission from the client computing device; a second module configured to, for respective electronic transmissions of the one or more electronic transmissions: scan content of the respective electronic transmission to identify one or more keywords included in an advertising data structure; select one or more advertisements for insertion into the respective electronic transmission based on relationships stored in the advertising data structure, wherein the stored relationships indicate associations between the identified one or more keywords in the respective electronic transmission and the one or more advertisements; insert the selected one or more advertisements into the respective electronic transmission, wherein inserting the selected one or more advertisements comprises changing at least one display characteristic of one or more words already included in a message body or subject line of the respective electronic transmission; and initiate transmission of the respective electronic transmission to one or more intended recipients. 18. The computing system of claim 17 , wherein said changing at least one display characteristic comprises one or more of changing a text color of the one or more words or underlining the one or more words.