The subject line cannot mislead the recipient about the contents or subject matter of the message. You have only 3 seconds to get their attention. Make sure it is 30-40 characters including spaces (5-8 words) It should be time sensitive.
This is a solicitation from XYZ. This email has been sent to you because you have previously contacted us or we have a prior business relationship. This email message has an opt-out provision. Once you opt out, your email address is removed from our database. XYZ will not share your email address with any other parties without your prior permission.
Day of week and time of day needs to be tested. There is a no exact time or day that works for everyone.
Do not use “Click here”
Do not use excessive punctuation