Hi, @Paul Siso
Depending on your current environment configuration, you will need a Client Access Licence (CAL). Every Exchange client that uses a mail server must have a CAL. You have two options, the user CAL or the device CAL.
Given that your users access webmail through five specified devices, you can select the device CAL, which will cover all five devices.
More information can be found Client Access Licenses (CAL) & Management Licenses | Microsoft Volume Licensing
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