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You should use push to add the users to the list

(all code untested - have never used nodemailer but this looks pretty basic)

for (var i=0; i<fromUser.length; i++)
    {
        maillist.push({email: fromUser[i].email, user_name: fromUser[i].Name }); 
    }

Drop these lines

maillist.toString();
users.toString();

Then put your mail.sendMail into a for loop

for(var m = 0; m < maillist.length; m++) {
    mail.sendMail({
            from: 'Website Support <help@domain.com>',
            to: maillist[m].email,
            subject: 'Seller Matched',
            template: 'SellerMatch',
            context: maillist[m]
        }, cb);
}

And change the handlebars in the template to reflect the new object from maillist