Use https://pqrs.org/osx/karabiner/ to disable Option+Return to send emails in Sparrow Mail

private.xml

<?xml version="1.0"?>
<root>

  <appdef>
    <appname>SPARROW</appname>
    <equal>com.sparrowmailapp.sparrowappcast</equal>
  </appdef>

  <item>
    <name>Sparrow disable Option + Return</name>
    <appendix>Disable Option + Return in Sparrow Mail</appendix>
    <identifier>private.sparrowmail_option_return</identifier>
    <only>SPARROW</only>
    <autogen>
      __KeyToKey__
      KeyCode::RETURN, MODIFIERFLAG_EITHER_LEFT_OR_RIGHT_COMMAND| ModifierFlag::NONE,
      KeyCode::VK_NONE,
    </autogen>
  </item>

</root>