This is not a “Weird” problem … it’s a rights issue in the “allowed to send” restrictions list.
The individual addresses and group addresses have different rights policies. Is there a “Trainee” distribution or security group that the user is sending to?
If so … go to the Active Directory snap-in ON THE EXCHANGE box and locate the group. Right click the group and go to properties. Under the “General” tab look at the “Message Restrictions” box and see if there are limits on who can send to the group. If so make adjustments as necessary.
It sounds as if you have “Accept messages only from” selected and this user is not in the list. If everyone should be able to send to this group then select the option for “All authenticated users” or “From everyone”.