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I was tired of guessing my way around why and by what measures Gmail clips the footer of an HTML email, so I finally looked it up.
First off, why don’t you want your email to get clipped?
Its tracking code will be clipped too, so your stats will be affected.
Usually, the footer also contains links for unsubscribing or managing subscription settings, and if those get hidden, it can become an annoyance for your subscribers, or even a privacy issue.
Gmail clips emails that have a message size larger than 102KB.
‘Message size’ here translates into the number of characters that make up the HTML code of your email. Each character of the code (not only of your content!) counts as ~1-2 bytes.
Images do not count for that 102KB limit because they’re loaded from external URLs; however, the HTML generated to embed an image does count. (It’s still a good idea to optimise your images before uploading them to Mailchimp.)
To avoid your email getting clipped, make sure its HTML source stays under 102KB. To check its file size, send a test email, copy its source code, and save it in a text file. This may seem tedious, but it can give you an overview of which parts generate larger chunks of HTML.