Go to Leases and find the lease where you want to communicate with the tenant. Click on the name of the rental object, and go to the Leases tab. Click on the name of the tenant, then "Lease log": 

Here you will find a log of everything that has happened in the tenancy. You can choose whether letter templates you download for the tenancy should be logged here. You do this in the settings in the rental module.
Scroll down the page to send an email to the tenant: 

In Message type you can choose between Note, Email and SMS. To send an email to the tenant, an email address must be entered for the tenant.
It is not possible to send SMS via the system, only to log messages already sent.
You can also log/enter general notes.