Starting today, you can use Amazon CloudWatch metrics and logs to get data and actionable insights from emails flowing through your Amazon WorkMail organization. Using CloudWatch metrics emitted by Amazon WorkMail, you will have an overview about the number of emails that are sent, received, or bounced. If CloudWatch logging is enabled, Amazon WorkMail publishes detailed CloudWatch log events about each email sent or received in your organization. You can then use CloudWatch Logs Insights to explore and analyze a wide range of information, such as delivery failure reasons, domains sending the most spam, or the number of emails bounced due to a misconfigured email flow rule.
from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/05/monitor-emails-in-your-workmail-organization-using-cloudwatch-me/