I'm getting the above error. According to the documentation, the daily quota is 50,000 email reads/writes. But my scripts/add-ons aren't doing anything heavy duty, and they're not processing many emails (two scripts processing a few dozen threads every five minutes). So there's no obvious optimization I can see.
Each script/add-on starts by doing a GmailApp.search() for threads to be processed. The number of emails being searched is large (thousands), but the number of emails found is small (10-20). Is it possible that the GmailApp.search() operation is using up quota?
Are there any other likely culprits?