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Yes. You can use almost any email client (Mail, Entourage, Eudora, MailSmith, PowerMail...) to create and send your messages.
You can also use some scripting additions, such as 24u's Email osax or JB Le Stang's XMail.
And finally, through "do shell script", use your built-in copy of sendmail and lots of tools available to other scripting languages (eg, Brandon Zehm's sendEmail, written in perl).