Really Good Emails - A large collection of good product email design.
SaaS Marketer - Biweekly, insanely actionable SaaS marketing case studies.
Good Sales Emails - Best sales emails from great companies
EnterpriseReady - A guide to building the SaaS features that enterprises love.
Great Email Copy - A dose of emails from around the web.
Zero to Marketing - Short case studies that help you get better at marketing