I cannot send emails

Hi, can anyone help me.

One day everything was find, the next day none of my emails would send, this applies to all my mailboxes.

I get the message;

 An error occurred
 Cannot send message (email recipient 
 not accepted by server.)

I have been through all the setting and cannot find the problem. This problem only arises on my desktop PC, mobile devices using a different client have no problems.

I would be so grateful for any help.

Many thanks, Tom,

Off the top of my head, have you tried temporarily closing your ant-virus, VPN software to see if that helps?