Description of problem:
At line 11 of /usr/share/perl5/vendor_perl/Email/Simple/Creator.pm, one can see:

  sub _date_header {
    require Email::Date::Format;
    Email::Date::Format::email_date();
  }

Perhaps it's because the requirement is declared inside a function that rpmbuild doesn't pick it up automatically?

However, it is required for proper operation of the package.

==== Example script ====
[mathieu@localhost ~]$ cat testmail.pl 
use strict;
use Email::Sender::Simple qw(sendmail);
use Email::Simple;
use Email::Simple::Creator;
use Email::Sender::Transport::SMTP;

# -- Change to appropriate values ------------------------
my $from = 'foo';
my $to   = 'foo';
my $smtp = '10.0.0.1';
# --------------------------------------------------------

open(my $fh, $0);
my $body = join("", <$fh>);
close($fh);

my $email = Email::Simple->create(
   header => [
     To      => $to,
     From    => $from,
     Subject => "Perl module to send emails",
   ],
   body => $body,
);

my $transport = Email::Sender::Transport::SMTP->new({
   host => $smtp,
   port => 25,
});

sendmail($email, { transport => $transport });
========================

==== Script output ====
[mathieu@localhost ~]$ perl testmail.pl
Can't locate Email/Date/Format.pm in @INC (@INC contains: /usr/local/lib64/perl5 /usr/local/share/perl5 /usr/lib64/perl5/vendor_perl /usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5 .) at /usr/share/perl5/vendor_perl/Email/Simple/Creator.pm line 12.
=======================

Once I manually install perl-Email-Date-Format, the above script sends the email properly.

Running repoquery with the --releasever parameter, I can see that the package is missing the requirement in all versions of Fedora (from 16 to 18) and EPEL (5 and 6).