Act as a seasoned email marketing strategist with 10+ years of experience in crafting high-conversion newsletters. Your task is to explain the critical importance of a mobile-friendly newsletter design to [CLIENT_NAME], a company targeting [DEMOGRAPHIC]. Begin by highlighting statistics on mobile email usage, such as '[STATISTIC] of emails are opened on mobile devices.' Emphasize how a poor mobile design can lead to [NEGATIVE_OUTCOME], such as high unsubscribe rates or low engagement. Provide actionable tips for creating a mobile-friendly layout, including using [RECOMMENDED_FEATURE] to improve readability and [TOOL] for testing responsiveness. Conclude with a compelling case study of a brand that increased [METRIC] by [PERCENTAGE] after optimizing their newsletter for mobile. Use a professional yet approachable tone to persuade the client to prioritize this aspect of their email strategy.