To do what you want, you should generate a MIME document. If you really want to do it right, create a multipart MIME document so you have both the TEXT and rich-text parts.
You can do it from Net::SMTP, but you have to add the necessary MIME header and part dividers to the document. See "Sending Email using Ruby - SMTP" for an example how.
It's easier to use the Mail gem, which supports both, especially if you're including multiple parts or adding attachments. From the documentation:
You can also create MIME emails. There are helper methods for making a multipart/alternate email for text/plain and text/html (the most common pair) and you can manually create any other type of MIME email.
And farther down in the document in "Writing and sending a multipart/alternative (html and text) email":
Mail makes some basic assumptions and makes doing the common thing as simple as possible.... (asking a lot from a mail library)
mail = Mail.deliver do
to 'nicolas@test.lindsaar.net.au'
from 'Mikel Lindsaar <mikel@test.lindsaar.net.au>'
subject 'First multipart email sent with Mail'
text_part do
body 'This is plain text'
end
html_part do
content_type 'text/html; charset=UTF-8'
body '<h1>This is HTML</h1>'
end
end