After analysing 250,000 emails, Boomerang came to the following three conclusions:
An 14% decrease in response rate was observed for emails with misspelt subject lines.
Second, the best response rate is seen in emails with a word count between 50 and 125.
50%
Third the highest response rate (53%) is for emails written at a third-grade reading level.
And lastly, subject lines for emails should only contain about three or four words at the most.
Source: https://buzzsumo.com/blog/7-effective-techniques-better-email-marketing/