Long emails are cut off in many large email clients such as Gmail, Yahoo and Outlook: Your audience doesn't see content past the cut off point.
Each email client has a different criteria when it comes to truncating messages. Some use total height (in pixels), some use code size (in lines of code) while others use message size (in kilobytes). Due to the various cut off rules there isn't a fixed maximum length to work with. Keep content brief, ideally no longer than a 3 or 4 minute read.
Long emails that are truncated are a source major issues:
Your audience doesn't see content past the cut off point. Sometimes they are presented with a "view entire message" link but not always.
Opens are not tracked for clipped messages because the tracking code at the bottom of emails is cut off. This skews your overall open rate for messages.
Some of your time invested in creating content and adding it to your email is essentially wasted as your recipients won't see the full message.
What you can do to prevent emails from being truncated?
Use "read more" links
Instead of adding long content directly inside emails, use "read more" links that take your contacts to pages that contain the full article.
"Read more" links have an added benefit in addition to helping with truncated emails: these links are trackable so you can tell how many people clicked through and this can guide your content strategy. In contrast, it's not possible to tell how many people read a longer article contained entirely inside your email.
Shorten your content
Keep content brief, ideally no longer than a 3-4 minute read. Emails are most effective when they provide enough information to readers so they become interested to click through and read the full article on a landing page.
Change the subject line of test messages (Gmail only)
Depending on recipients' settings, Gmail can display messages in threads that use the exact same subject line. Threads can grow long and this can result in messages being clipped. Changing the subject line when you're sending many test messages over and over prevents emails from shown in threads and instead each email is shown as a unique new message.
Related article: Troubleshooting email display issues