The last one is the case for Exchange Server licenses for Office 365, Microsoft 365 and Exchange Online hybrid deployments:
When Microsoft gives you something for free, they either want something back or set restrictions. Hence, when you currently use Exchange Server 2010, you might even upgrade to Exchange Server 2016 Standard without buying a new software license. If you are migrating your on premises e-mail and calendar functionality from Exchange to Exchange online, you can use your existing Exchange Server as your coexistence server. Even better, when you want multiple Exchange Hybrid Servers – and you want for availability reasons – you can use the same software license key for those multiple servers. You do not need a separate payed software license for this on premises Exchange Server.
This is the Exchange Server Edition you will get for free when you deploy a hybrid Exchange environment through the Exchange Hybrid Configuration Wizard. Using Software Asset Management services or tooling, organizations might detect an Exchange Server edition called ‘coexistence’. The answer to the question is ‘No, you do not need a license for your Exchange Server Hybrid’, but off course there are some restrictions. But what in hybrid scenario’s? Do you need a software license for your Exchange Server in hybrid mode? With the modern cloud scenario’s using Office 365 or Exchange Online Services, each user for the service needs a User Subscription License. In classic scenarios organizations need a software license for each Microsoft Exchange Server they deploy, accomplished with access licenses for users or devices.