change_to_line.rb

mime_message = DATA.read

puts "To line before: "
puts '  ' + mime_message.match(/^(To\:.*)$/i)[1]

# Massage the To: line.
changed_to = 'CHANGED@example.com'
mime_message.gsub!(/^To\:.*$/i, "To: #{changed_to}")

puts "To line after: "
puts '  ' + mime_message.match(/^(To\:.*)$/)[1]

# Remove the BCC: line.
mime_message.gsub!(/^BCC\:.*\n/,'')
puts "Removed BCC:"
puts mime_message

__END__
BCC: support@ourcompany.com
To: Guest <guest@example.com>
From: Some One <someone@example.com>
MIME-Version: 1.0
Content-Type: multipart/mixed;
        boundary="XXXXboundary text"

This is a multipart message in MIME format.

--XXXXboundary text
Content-Type: text/plain

this is the body text

--XXXXboundary text
Content-Type: text/plain;
Content-Disposition: attachment;
        filename="test.txt"

this is the attachment text

--XXXXboundary text--