This gives even LESS control over the look of the email...
From original post: "I want all fonts to look the same regardless of who the receiver is."
In my opinion, using plain-text gives
absolute control over the "look" of the email -- it'll be words and sentences, and the recipient's system will display exactly those words and sentences.
Having an email look the same across the board, regardless of receiver, regardless of platform, regardless of email client is absolutely impossible, just like making a web page look exactly the same on all browsers, on all machines, on all platforms is impossible (unless you use nothing but images and strictly-defined tables, which is unacceptable web design, and even then, text-based browsers like lynx won't display the images).
Even if you use HTML emails and strict font declarations, there are recipients out there that have their email clients configured to override these settings (like me -- absolutely no HTML emails, because email was never designed for "beautification" with HTML, and if I wanted to look at something pretty in HTML, the sender should send me a web page link -- not embed the web page in an email).
Not to mention that not all systems use the same fonts -- there isn't a single font in the world that is guaranteed to exist in the same, exact format on Linux, UNIX, Mac OS X nor Windows. Sure, you can pick a font that might be present on 80% of the recipients you choose to send to, but there isn't any way at all to guarantee that when you use a certain font that it exists on the recipient's system.
The short answer is that it is impossible to guarantee exactly how the email will look on the recipient's end. Using nothing but plain-text, though, gives you the most "purity" of email, ensures that the actual content of the email is exactly what you intended, and using plain-text ensures that nothing is lost in the translation and transmission of the email. What you type is what they receive, and if they want to read emails with Courier 12pt instead of Monaco 10pt, that's the end-user's decision -- not the sender's.
I realize that HTML email is so common now that it's almost taken for granted, but it's a hack and a bolt-on to the original intended purpose of email. It would be a great advantage, though, if you intend on sending "pretty" email, to understand that email was never designed for that kind of content in the first place, and attempts to do so will never satisfy 100% of the recipients (although most -- not all -- email clients will display HTML emails
similarly). The only guaranteed thing with email is the text itself, and, while this is my own opinion (and, also, the opinion of the inventors of what we know as email), the sender should not be dictating the format of the text that appears on the recipient's end.
Just a few cents worth of defense and rambling there; FWIW.