A Google search of "smtp multiple attachments" returns about 188,000 hits. The first page pretty much has a number of packages to do this, all of which require some sort of payment. I suspect your requirement cannot be met without purchasing a package. However, I've never tried sending multiple attachments through SMTP so I have no idea about whether or not it can be done except through these packages.
You could use tar or pax (or even GZIP, if it is installed at your site) to create a single file and attach it to your email.