This patent explains the different ways the iPhone could filter text based communications in your iPhone. Apple says the main use for this is, to censor objectionable text like swear words in email for children, or to help someone learn a foreign language by forcing someone to send emails in that language.
…control application includes an instructional tool or study aid where the administrator sets one or more modes, such as language, vocabulary, grammar, spelling, punctuation and/or other content of a text-based communication based on, for example, a user’s age or grade level. This can be especially useful, for example, such as when a child’s grades go down. A parent can then institute a condition to improve a child’s grades. For example, the control application may require a user during specified time periods to send messages in a designated foreign language, to include certain designated vocabulary words, or to use proper designated spelling, designated grammar and designated punctuation and like designated language forms based on the user’s defined skill level and/or designated language skill rating. If the text-based communication fails to include the required language or format, the control application may alert the user and/or the administrator/parent of the absence of such text.