Problems with Outlook sending emails, sometimes!

Have a client’s office where mainly one pc exhibits random failures to send email. Peer to peer small network, Outlook 2003, have used used 3 smtp servers, changed from Norton to Kaspersky with the ‘check outgoing emails’ set to off. when sending group email the no. of recipients is below the ISPs limit, Business strength broadband. When I am called in it doesn’t happen! I have to leave the support to other staff after the 26th and really need ideas to sort, it’s been happening for some time. Any ideas?