Refactor EMAIL_MAILBOX > handleNewMessageEvent && fetchEmailBody methods

**Is your feature request related to a problem? Please describe.**
Right now these functions are cumbersome, crowded and prone to errors. Example, fetchEmailBody will return undefined if the request status is not 200 or 499.
handleNewMessageEvent object dereferences the return value of an undefined throwing a wrong exception.

**Describe the solution you'd like**
This needs to be broken down into simpler functions and logic

**Describe alternatives you've considered (Optional)**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context (Optional)**
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