[RCU] obtaining complete Email Headers with IP addresses

If anyone knows how to obtain the IP address of sender's of emails when both users are Roundcube email users, please let me know.  I am trying to determine the actual IP address of the senders of emails.  We are both Roundcube users but we have different internet service providers.  Roundcube does have a facility to show some Header source but it does not show actual IP addresses.  Is there some way to determine the actual IP address from whence the sender sent the email when they were using the Roundcube system and sending their email to another Roundcube user?

Any help is very greatly appreciated.

David