Net::Cmd->code()
Net::Cmd->message()

##</code><code>##

# Create ourselves an SMTP object
my $smtp = Net::SMTP->new(
   Host => 'some.mailserver.de', 
   Hello => 'my.mailserver.co.za', 
   Timeout => 30,
   Debug => 0);
# Talk to the server
$smtp->mail("postmaster\@mailserver.co.za", Size => 20971520)
   or die "Oops. Server said ", $smtp->code(), " ", $smtp->message();
$smtp->to("youthere\@mailserver.de")
   or die "Error on RCPT TO: ", $smtp->code(), " ", $smtp->message();
$smtp->quit;

##</code><code>##

$smtp->mail("postmaster\@mailserver.co.za", Size => 52428800);
if ( $smtp->status() =~ /^[45]$/ ) {
   # informative error here
   exit(1);
}

##</code><code>##

if ( $smtp->status() =~ /^[45]$/ ) {
   if ( $smtp->message() eq "size limit exceeded" ) {
      print "Server returned a fatal error:\n";
      print $smtp->code(), " ", $smtp->message(), "\n";
      print "The message is too big. Boo-hoo\n";
   }
}