With Oempro v4.3 update, you can now add your own custom email headers into the source of all emails sent from your Oempro. This feature brings several advantages. One of them is the ability of segmenting your sending on the MTA level. You can insert user group id into your outgoing emails and then balance your sending through virtual MTAs setup in your MTA, use different IP blocks for each each user or send transactional emails through different IP blocks.