WHMCS - Send mail to client

Hi, My name is John And i have been using whmcs a quite time now.
So my issue is that when i try to send a mail to one of my clients i get this error:
3lmRVD3.png
But it also doesnt work for example when you are going to reset your password or sending invoices to my clients it doesnt work.

I really need help with this fast.

Here's my settings:
5lzrE3A.png

Attached Images

• 3lmRVD3.png (52.6 KB)
• 5lzrE3A.png (107.7 KB)