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As large messaging providers increasingly adopt end-to-end encryption, private communication is readily available to more users than ever before. However, misunderstandings of end-to-end encryption's benefits and shortcomings limit people's ability to make informed choices about how and when to use these services. This paper explores the potential of using short educational messages, built into messaging workflows, to improve users' functional mental models of secure communication. A preliminary survey study (n=461) finds that such messages, when used in isolation, can effectively improve understanding of several key concepts. We then conduct a longitudinal study (n=61) to test these messages in a more realistic environment: embedded into a secure messaging app. In this second study, we do not find statistically significant evidence of improvement in mental models; however, qualitative evidence from participant interviews suggests that if made more salient, such messages could have potential to improve users' understanding.