I've only heard of this with received mails, not sent ones. Anyway it's worth trying this. Ask your brother to go to Tools/Options, Security tab in OE and take the tick out of "Do not allow access...". Then try again.
Samsung Galaxy S8 review
1995-2015: How technology has changed the world in 20 years
This 2D animation about monsters is illustrated in a Van Gogh painterly style
How to get WWDC 2017 tickets
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.