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Setting up a Mail Handler requires a Profile to be created first, the following will be shown otherwise:
Navigate to Messaging > Message Handlers and click Create:
Fill in the blanks, giving a name, selecting the Profile and Inbound Message Source:
NOTE that scheduled retrieval of messages for a given Mail Handler can be disabled here!
Once saved, a summary is shown:
There is a Poll action 
which is effectively a manual invocation of the Mail Handler.