I had the same problem, and that was because I use Handlebars as a merge when sending emails.
Mailchimp places the MC_PREVIEW_TEXT variable in the template using the Mailchimp merge language, so if you use Handlebars, it appears.
To fix this, you should set your Merge Language to Handlebars in the Mandrill settings β Send by default.
But when you do this, you must use the Mailchimp merge language when designing your letters in Mailchimp, you cannot use Handlebars.
Then, when you send Mandrill from Mailchimp, it will convert all your merge variables to Handlebars.
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