Hi ML,
For your scenario, your web app can generate an XML file that contains the specific parameters (with values) and save it in the folder where your BizTalk process will pick it up and route your message based on those parameters. You will have to define an incoming schema for the message that triggers the BizTalk process, and promote the fields in that schema (that are mapped to the parameters) so that you can filter on them in a Send port that uses the SMTP adapter to send email to users.
I assume that you are contemplating a messaging-only scenario, so you will need to define a schema in a BizTalk project, promote the fields that you need to do routing based on their values, validate the schema and publish it. Then, in the BizTalk Administration console, you will create a Receive Port with a Receive location (using standard File Adapter) which will receive your XML file, and create a Send Port (that uses SMTP adapter), specify the Filter expressions in the Filters tab (where you reference the promoted properties of your schema), configure the adapter settings (e.g. email addresses in To and CC fields, SMTP Server name, etc.) and have it subscribe to the Receive Port.
I hope this is enough information to get you going.
Daniel.