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If you plan to increase your company's Twitter presence, promoting your tweets in emails is a very effective way to do that.
Click on + Add Twitter button
Add your Twitter username
Click Save and you should see your latest tweet.
The app lists only tweets, not retweets and replies; you don't need to worry that the content appended to your emails will appear out of context.
However, there are also some ramifications - if your retweets and replies outweigh your original tweets in a major way (more specifically: if your 200 Twitter interactions consist solely of retweets and replies), no content will be loaded.
Please keep in mind that External sources work best when used with Subscription - a synchronization will be performed before every set-up that will fetch the most current content.