Length of subject lines
In many inboxes subject lines get trimmed after about 60 characters, depending on the email app. The technical limit is 255 characters in Envoke.
Emojis in subject lines?
Yes, you can use emojis in subject lines. Read more here.
Subject lines are not alone
The pre-header content of your email is an extension of the subject line. Always think of them together and make sure they complement each other. Read more about pre-header content here.