Outlook issue with public folder send as

Hello all,

First things first, Merry Christmas.

Now onto the problem, I recently created a new mail enabled public folder on our exchange 2010 server and added 4 people to the send as list. Now 3 of the 4 people can send as this address no problem. One user on the other hand is having quite a weird issue.

If he adds the account to his phone then he can send as from it, if he uses OWA he can send as it but if he tries using outlook on any PC to do it then it bounces back.

Does anyone know how to fix this?