IMAP Content Sensor
The IMAP content sensor seems to be a little finnicky when getting set up. The errors that show up are less than useful. Listed here are some things I ran into and how to fix them.
Setting up with GMAIL
If you have 2-Factor Auth set up, you need to use an app password.
Setting up with AOL Mail
Initially I had issues getting the sensor to work, even with the correcty IMAP server set and my username and password. There is an additional setting under your account settings in "Security" where you need to all less secure logins.
Sensor unable to update
I had an issue where the emails it was looking for did not have a subject. This caused the sensor to fail updating.