The report was based on an analysis of 250,000 campaigns sent by brands in 2016. The researchers passed the messages through an automated grammar and sentiment checking tool to measure how error rates and tone change over the course of the week.
Campaigns sent on Mondays have the most errors (spelling mistakes, grammatical issues, improper capitalization, etc.) in subject lines, on average.
Emails sent on Fridays have the second-most errors in subject lines, on average.