Mailing list content must be ultra-short. Provide separate email addresses for subscribing and unsubscribing and include info on how to get off in every mailing list message. Mailing lists allow you to extend the footprint of your website .
Some companies send out very long emails that constitute complete newsletters.
I prefer keeping the email very short , restricting it to headlines and summaries with links to more extensive content on the website. The rule for Web content is keep it short. The rule for email content is keep it ultra-short . You want the email to link back to your website since that's where you can provide a much richer user experience in those cases where the user does want more depth than can be provided in an email. Users would send the server a list of commands in the body of the email message, much as they used to submit a batch of punched cards to the mainframe priests.