WP Email Manager enables you to limit the number of email accounts that can be created on a global basis i.e. applies to all plans.
To enable this functionality, go to the plugin’s setting page in the administration area and enable WP Ultimo integration. Once done, you will be able to configure the maximum email accounts and maximum disk space per account allowed for all plans.
If you would like to override the email account limitations per site, refer to this article.
NOTE: We don’t have plans to enable limiting email accounts per plan for WP Ultimo version 1 .x since the plan limitations are attached to a user, not the site itself.