I’ve found that Mail.app always sends in plain text if possible, no matter how you have your preferences for “Message Format” and “Use the same message format as the original”, on its “Composing” preferences tab.
Basically, it lets you compose in rich text mode (if your settings allow that), and then afterwards, looks at what you’ve typed to see if it thinks anything would be lost by sending in plain text mode. If not, it uses plain text, but if so, it’ll then send both HTML and plain text (with Content-Type: multipart/alternative). That’s why you can compose in rich text mode, but end up sending in plain text – Mail.app is waiting to see if you’re going to use any formatting that it must preserve.
Personally I think it would be better to always send HTML if you’re composing in rich text mode, or always make you compose in plain text mode if it’s going to send plain text, but Mail.app is trying to be “smart”, so this is how it works out. Thanks, Apple. :-P
So anyway, you can force it to send HTML by styling some of your text – for instance, making a word or two bold, or using a colored font somewhere (including in your signature). And if you are replying to or forwarding to content that contains HTML styling, your message will be sent in HTML in that case too.
However, even when sending HTML, Mail.app doesn’t set the font, so whatever the default font is in your recipient’s mail client, that’s what they’ll see (with bolding and/or colors applied to it, etc). This is the problem the original question was about. Outlook happens to default to Times New Roman, so that’s why Outlook users will see your message that way; most webmail users will see some sans-serif font like Arial, in my experience (I haven’t bothered to look into how much that is due to the webmail system and how much is just because my browsers default to Arial).
But! Mail.app does set the font for your signature, as long as you haven’t checked the “Always match my default message font” checkbox. So one hacky way I’ve found to send my messages with Verdana is to make my signature like this (choosing Verdana as its font):
Hello,
-jason
When composing/replying, I then delete the blank line above “Hello”, maybe add the recipient’s name after or in place of “Hello” itself, then add a few blank lines between that and my name, and type my message as normal. When I send, the font information is included.
In essence, my entire message is within what Mail.app thinks of as my signature. Yeah, I said it was hacky, didn’t I? :) But it works.
However, two caveats: One, the header at the top of the quoted text (“On [date], [so-and-so] wrote:” still appears in the recipient’s default font. And two, this really bloats up the size of my messages (use View > Message > Raw Source to see what I mean).
But.. eh. It’s the best solution I’ve found so far.