The length of an email should be proportional to how well you know the recipient, with shorter emails for those you know less. The email should state the reason for contact, then list 3-5 benefits of a reply supported by data. Avoid a meaningless intro and make the scope clear immediately, starting with the purpose for writing. Personalize the email by showing you know the recipient and addressing their current issues.

1. How to write an email
The length of the email is directly proportional to
how well you know the recipient. The less you know
the person you are writing to, the shorter the email
should be.
1 - LENGTH
After stating the reason behind your email, validate
why they should reply by listing 3-5 benefits.
Support your claims with data and actual numbers.
3 - STRUCTURE
Avoid the meaningless intro. Start with "I'm writing to
ask/enquiry/learn/discuss..." Make immediately clear
the scope of your email.
2 - INTRO
Show you know the person you are writing to. Your
proposition should solve the issues they are currently
facing.
4 - PERSONAL TOUCH