The Outgoing Mail Control is set to Unlimited (on both subscription and domain level), but still not possible to send mail messages.
CONFIG_TEXT: 12/15/17 13:17:56 ME-I0xxx: The 2 recipient(s) for mailbox <example.com> puts it over limit of 1000 per hour (current count 1000).
12/15/17 13:17:56 ME-I0xxx: Postoffice example.com user user@example.com has sent too many messages this hour and message rejected.
This is a bug with ID PPPM-7749 : MailEnable limits are not adjusted according to Outgoing Mail Control settings.
MailEnableAdmin > Server > Services and Connector > right-click on SMTP > Properties > Security tab.