I have potentially discovered in the latest build of Mail.app (Version 3.6 (935/935.3)) mail is sent in Plain Text only with no HTML, despite the default compose setting that Rich Text is set - unless you make a slight alteration to the composition from the default text.
Example, I compose a new message and verify the Format > Shows 'Make Plain Text' signifying it is currently using Rich Text. Mail.app preferences has the Compose options to use Rich Text as well. I write a few lines in the new mail window and even drag and drop a picture 'in line' which is how I want the recipient to see the mail. If I do not make any changes to the text in terms of formatting (changing the font, style or size), when I send the message, it will send as Plain Text only - it does not include the HTML equivalent, therefore the recipient only sees the Plain Text message and will see an attachment of the picture - it does not show 'in line' as intended. I verify this by checking the message source on the recipient account. I also have the option to display plain text messages with a fixed with font in Mail.app - so it is easy to see if the message in the 'sent' folder was sent in plain or rich text.
Now, if I repeat the above example, but select any part of the text and make a slight adjustment, even if I were to change the default font size from 12, to 13, then immediately back to 12, that seems to be enough to trigger a flag somewhere to indicate this is indeed a Rich Text mail. So when sent, it is sent as both Plain Text and Rich Text/HTML, arriving as intended to the recipient, with the picture displaying 'in line'.
So in short, even though I have set my compose preferences to send as Rich Text by default, this is ignored unless I make a slight adjustment to the text (and back again!) before sending.
Example, I compose a new message and verify the Format > Shows 'Make Plain Text' signifying it is currently using Rich Text. Mail.app preferences has the Compose options to use Rich Text as well. I write a few lines in the new mail window and even drag and drop a picture 'in line' which is how I want the recipient to see the mail. If I do not make any changes to the text in terms of formatting (changing the font, style or size), when I send the message, it will send as Plain Text only - it does not include the HTML equivalent, therefore the recipient only sees the Plain Text message and will see an attachment of the picture - it does not show 'in line' as intended. I verify this by checking the message source on the recipient account. I also have the option to display plain text messages with a fixed with font in Mail.app - so it is easy to see if the message in the 'sent' folder was sent in plain or rich text.
Now, if I repeat the above example, but select any part of the text and make a slight adjustment, even if I were to change the default font size from 12, to 13, then immediately back to 12, that seems to be enough to trigger a flag somewhere to indicate this is indeed a Rich Text mail. So when sent, it is sent as both Plain Text and Rich Text/HTML, arriving as intended to the recipient, with the picture displaying 'in line'.
So in short, even though I have set my compose preferences to send as Rich Text by default, this is ignored unless I make a slight adjustment to the text (and back again!) before sending.