How much limit per user.

Hi,

you can find all relevant quotas and limits for WorkMail here: https://docs.aws.amazon.com/workmail/latest/adminguide/workmail_limits.html

Recipients addressed per user per day using the domain that you own: 10,000 recipients external to the organization, with no hard quota on recipients internal to the organization. There is no upper bound. However, Amazon WorkMail is a business email service and not intended to be used for bulk email services. For bulk email services, see Amazon SES or Amazon Pinpoint.

In short, every user is allowed to address up to 10,000 recipients per day by default. Please not the terms and conditions of the service regarding bulk email services.