I think for two options, either to send messages to Drupal with SOAP or XML-RPC or by sending e-mail.
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165answers:
4If you want to post by mail, check out the Mailhandler module. It doesn't look very popular but it sounds like someone new is taking over maintenance.
If you want to do it via XML-RPC, Drupal has a built-in XML-RPC interface but I don't know about using Outlook for an XML-RPC client.
From the description, it looks like this module does exactly what you want. From the page just linked:
"Mobile Media Blog allows you to post media via e-mail or mobile phone. It integrates with Mailhandler and saves the submitted text as node and attaches all files sent with it. You may choose to use the inline module to display attached files automatically. The mailalias module allows your users to provide more than on e-mail address for posting."
ORIGINAL POSTER SPECIFIED HE/SHE WANTS TO USE WEBSERVICES
If you want to use webservices to achieve this functionality, then I would suggest to use the blog_api module from the Drupal core. This module essentially exposes a number of well-known standards via the xml-rpc interface (MoveableType is the oldest one, but there are also MetaWeblog e Blogger API's in there).
Don't let you be fooled by the name of the module: with *blog_api* you can actually interact with any type of nodes on your site.
As for the reference material, that depends by the kind of API you want to adopt. Here is an example for the blogger API.
On a side note: if you go for the blog_api solution, then it might be useful for you to know that there are a number of blogging clients available as desktop applications under nearly any possible operating system. You might well consider using a dedicated software then, rather then an Outlook add-on.
And of course, yet another module: Mail2Web. If you are using OG, Mail to OG may also be of interest.
Thank you all for your replies .. it was very helpful but all of them are using e-mails to send messages. What about web services? Drupal uses XML-RPC by default to send messages over HTTP but i can't find any useful book, report or even a tutorial that shows a way to use XML-RPC with Outlook. Any information is welcome. Thank you.