Place the domain name in your safe senders list which you can find in the settings by clicking on your username at the right hand side. Once there click on Mail then Safe Senders and at to the list and click save.
If possible are you able to establish if any other BTMail users in the mail group are experiencing the same problem.
Can you bring this link to the attention of the administrator of the email domain that you are having a problem with just ensure that they are using best practice.
