System.EmailException: SendEmail failed. First exception on row 0; first error: INSUFFICIENT_ACCESS_OR_READONLY, Not profiled to access this Org-wide Email Address:

Problem: Whenever you are referring the organization wide default address while sending emails through sendEmail methods,sometimes you may encounter this error.

Solution: Go to Organization Wide Email Addresses and give necessary profile level access.
Go to Setup --> Email Adminstration --> Organization-wide addresses --> click on 'edit' button of that Email and give profile level access.