1. Integration with Gmail: Boomerang is specifically designed for Gmail, providing tight integration with its interface and allowing users to schedule emails, set reminders, and track responses seamlessly. In contrast, GroupMailer is a standalone email marketing tool that is not integrated with any email platform, which may require users to manage multiple tools for different purposes.

2. Email Automation Capabilities: While Boomerang focuses on personal email management and scheduling, GroupMailer specializes in automated email campaigns, allowing users to create personalized drip campaigns, A/B test emails, and analyze the performance of campaigns through detailed reports. This difference makes Boomerang more suitable for individual email productivity, while GroupMailer is ideal for businesses looking to streamline their email marketing efforts.

3. Collaboration Features: Boomerang offers collaboration features such as shared email inboxes and customer management, making it suitable for small teams or individuals working together on projects. On the other hand, GroupMailer lacks such collaboration features and is more geared towards solo users or marketing teams focused on email campaigns rather than day-to-day email communication and coordination.

4. Customization Options: GroupMailer provides a wide range of email templates and design options for creating visually appealing campaigns, offering more customization features compared to Boomerang, which primarily focuses on email scheduling, tracking, and follow-up functionalities. This difference caters to users with different preferences in terms of email design and branding.

5. Recipient Management: GroupMailer offers advanced recipient management features such as segmentation, tagging, and personalized content delivery based on user behavior or demographics. In contrast, Boomerang lacks these advanced recipient management capabilities and is more focused on individual email organization and productivity, making it less suitable for targeted email marketing campaigns.

6. Pricing Structure: Boomerang follows a subscription-based pricing model with different plans based on individual or team requirements, offering a free trial for users to test its features. In comparison, GroupMailer provides pricing plans based on the number of contacts or emails sent per month, with additional costs for advanced features such as A/B testing or automation, making it a more cost-effective choice for email marketing purposes.

In Summary, Boomerang and GroupMailer differ in terms of integration with Gmail, email automation capabilities, collaboration features, customization options, recipient management, and pricing structure, catering to different user needs and preferences in email management and marketing.