Well, I don't remember very much about it, but I remember about PGP/MIME option.
To compose in HTML:
menu: tools > account settings > choose your account > OpenPGP security > Check Allways use PGP/MIME
menu: tools > options > compose > sending format > "send with a plain text version together" (something like that; mine is in Portuguese)
To read HTML:
From http://www.e-ignite.co.uk/html/openpgp.html :
PGP/MIME - when to use it:
PGP/MIME is a method for encryption and signing of email. It allows you to use text formatting so you can send html emails or sign and encrypt emails with attachments. If you are simply sending a text email, Enigmail OpenPGP saves the email as an html file, then signs and/or encrypts the file rather than the text. It is an advanced feature and has excellent functionality, however, not all email clients support PGP/MIME. If your intended recepient uses Enigmail, you are able to send PGP/MIME email to them. Otherwise, you would need to check with them to see if they are able to read PGP/MIME emails. Normally, Enigmail encrypts plain text emails only, so if you compose an html email (for example, using Bold style, a specified font, a font colour or a font size) then you will get a warning box asking if you want to remove the html formatting or if you want to use PGP/MIME instead. If you do not want to use PGP/MIME, you should compose your emails in plain text only by holding down SHIFT when you click "Write" to compose an email. Converting from html to plain text after composing the mail can cause problems with bad signatures.
Inline PGP is the most universally accepted method to use, however this is changing. As far as I'm aware, the only mainstream mail clients that do not support PGP/MIME are Microsoft Outlook and Outlook Express. If you sign an email and send it to an Outlook or Outlook Express user, they will receive a blank email with two attachments - the html message and a signature. They will still be able to read the message, but they have to open the attachment as a file because it will not display automatically within the program. There is an excellent list of email clients that support PGP/MIME and I'd recommend that you have a look at it here. If you and all your contacts use Thunderbird and Enigmail, there is no reason not to use PGP/MIME.