While sending your email campaigns, if you choose multipart the application will take both the combined form of HTML and Text version. When your contact receives this email, their email service provider like Outlook or Google will read it and display the most appropriate version which will be convenient for their client's device.
Even if you do not allow your contacts to choose either of the versions, the best option is to send your emails as multipart. Why because, if your client view your newsletter with a webmail account, they would be able to see the text version of your newsletter. If they view it in Microsoft's outlook then they would be able to see HTML version.
If your contacts are given the option to choose either of the versions then it is better to send both the types of email (Text and HTML) but not multipart. The application will see to it what your contact has chose to receive and will display them as required.
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