This paper is a mixed-method study divided into three parts. Part I provides an overview of the current literature on corporate greenwashing, social media marketing and the intersection of the two. The section concludes by developing a theoretical model and testable hypotheses that aim to explain how individuals might process tweets as it pertains to environmental messaging from external third-party commentators. Part II empirically tests the hypotheses through a 2x2 between-participants survey experiment (n=200) to measure the effects of information source (author of the tweet) and post message (positive versus negative) on two dependent variable constructs: post shareability and environmental authenticity. The empirical study finds that (1) positive posts have a higher likelihood of being liked/shared than negative posts, (2) that third-party commentary from more credible information sources are ranked more favourably in environmental authenticity for both positive and negative conditions compared to less credible information sources, and (3) that the credibility of an information source does not have a statistically significant effect on the likelihood of a post being shared or liked. Part III aims to provide colour into potential behavioural mechanisms behind the causal relationships tested in Part II. Part III is exploratory in nature and uses a series of qualitative in-depth individual case studies (n=10) to propose a set of future research questions that would develop a more nuanced understanding to complement the empirical findings.