Newsletters are sent with simple text version
There are two types of formats of email: HTML or plaintext.
MailPoet sends both versions, but you only see the colorful HTML version. Some email clients, such as some Blackberries, only show the plaintext version. Otherwise, it's hidden from you, unless you see the source or original email.
Spam filters also like to see a plaintext version with the HTML email. Spammers often don't bother creating a it.
How to view the plaintext version? Each email client as a menu for that. We won't list them here, but a quick search in the Help menu of your email client or in Google will do.
For example, in Gmail, you can find it like this: