spec: cleaned up specs around incoming mail

fixes CNVS-12162

QA Test Plan:
- run tests

Change-Id: I589cb777dbcf7207bf61b820674efe86e0f6dcbd
Reviewed-on: https://gerrit.instructure.com/32704
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>