BoltMail
What is multipart sending and why should I do this?
When you are sending an email you can choose to send it as "Multipart".

What this means is that the application will take both the HTML version and the Text version of your email and bundle it into the one email. When your contact receives this email their email client (such as Outlook, Gmail, Hotmail) will then read the email and display the most appropriate email for them.

If you do not allow your contacts to choose their subscription type when they subscribe and they are signed up as HTML then you should send your emails as multipart. The reason for this is that if your contacts log into a webmail client for instance, they will be shown the text version of your email. However if they then view their email in a client such as Microsoft's Outlook they will see the HTML version of this email.

If your contacts are given the choice between Text and HTML when they subscribe then you should still write both types of your email (Text and HTML) but not send using multipart. The application will then look at what your contact has elected to receive and send them an email of that type.