It inherits from IO::Handle, so something like this ought to work. (I've tested it, but the delivery delays here are terrible.)

#!/usr/bin/perl -w

use strict;

use Mail::Mailer;

my $mailer = new Mail::Mailer 'smtp', Server => 'smtp.server.net';

$mailer->open( { from => 'chromatic', to => 'chromatic@perlmonks.org',
+ subject => 'Not Valid Addresses' });
print $mailer "This is my text.";
[download]

$mailer acts like a filehandle in this case, so it ought to work with write().