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.
Surface Pro (2017) vs Surface Pro 4
20 groundbreaking 3D animation techniques
How to mine Bitcoin on Mac
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.