INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY When trying to send an email

Question

I've looked this error up, and it seems to be related to inserting updating records. I'm not trying to do anything like that. I'm simply using Messaging.sendEmail(). I'm passing it a list of Messaging.SingleEmailMessage. When debugging it appears that this list is properly populated. What could be causing this error when trying to send messages?

If it helps at all I'm calling this through a Rest API endpoint I've written in a without sharing class.

Answer 1

After lots of digging we figured it out. The code uses a function that down the line calls another function defined in another class. We needed to give access to our portal profiles to that class. That done, and it works fine now.