The correct answer is 'Software as a Service (SaaS) for email'. This is because SaaS solutions for email are designed to be easily integrated with other cloud services such as calendar and contact management. They generally offer a complete package that includes maintenance and support, making them an efficient solution for companies looking to leverage cloud capabilities without managing the underlying infrastructure. Implementing Infrastructure as a Service (IaaS) would require the company to manage the email server infrastructure themselves, which does not fit the requirement of seamless integration with existing cloud services. Platform as a Service (PaaS) typically requires development and does not inherently provide email service. A Custom Email API would require extensive development and management, and SMTP Relay, while necessary for sending emails, does not account for the integration with calendar and contact services.