Once you've identified a SMTP provider to use (which I can't help you with), an easy way to construct your email with separate plaintext and HTML parts would be by writing your own code for it.
Many programming languages offer support for this sort of thing; Python might be an easy choice because (a) it's probably already installed on OS X, and (b) someone already did all the work for you:
http://docs.python.org/2/library/email-examples.html
Check out the very last example on that page. It does literally exactly what you want. I myself run a version of that code every day. All you have to do is change the email addresses and the SMTP server near the bottom. (And the text of the email, of course!)
If you've never programmed in Python before, just copy that code verbatim into a text file, edit the file to your heart's content, and then type
python /path/to/your/textFile
in the Terminal to execute it.
I didn't downvote you, but I don't understand what you want; what does “to customise” HTML mean? Nor can I think of any “trusted third party SMTP host” that would relay email; that’s kinda why they are “trusted”. – Dour High Arch – 2013-08-15T14:58:31.007
@Dour, thanks for the reply. By "customise" I mean I would like to have control of both the html and plain text parts of the email, and not have the plain text generated from the html. I don't want to relay an email through a third party, I want them to send it for me. – pingu – 2013-08-15T16:23:56.347
@Dour - I have since found a windows application that does what I am looking for, anything for mac though? http://group-mail.com/html-email/how-to-create-a-multi-part-mime-email-html-and-plain-text/
– pingu – 2013-08-15T16:25:38.790