I am amazed at how many people have switched to sending html only emails. There are a few problems with this, most importantly is some mail servers will remove content if it is html, so you will end up sending a blank email to your user if you send it as html only. Also for some viewers with text only email readers (yes they do exist) the mails will look terrible if you send them in html format.
Sending multipart emails is easy, here are directions in my 3 favorite language/platforms:
Ruby on Rails - note how you can just define an html view and a text view and rails will figure out that you want to send a multipart email.
.NET / C# - just add each view to the mailmessage object before sending.
Java - you just add the parts to a MimeMessage.
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