Long emails are clipped in some email clients such as Gmail, Yahoo and Outlook. You can still send a long email but we suggest you shorten it to ensure your audience sees the entire message.
When emails are trimmed:
Your audience doesn't see content past the cutoff 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.
How to reduce the size of emails?
Shorten your content
Having many content blocks has the most effect in overall email size. 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.
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.
Media in emails doesn't affect email size by a lot
Images, videos and attachments don't significantly contribute to message size since emails themselves only contain a link to the files - the actual files are hosted elsewhere.
Test messages in Gmail
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