doc and xls file attachments not attached when received by recipie

[JRH]

When sending office files as attachments, sometimes the attachment is
received by the recipient and other times the email is received without the
attachment. This occurs when sending to only one addressee or multiple
addressees. The operating system of the recipient seems irrelevant to the
problem. If I open the sent email on my pc, the attachment appears and can be
opened.

Please send help.

I am using XP Professional with Office 2002