Won’t recognize an address in the “TO” block in an email

I log onto my work email here at home on my laptop with windows 7.  I can open and read my work email with no problem.  But if I try to respond to some one with an email.  it tells me I "must fill in an addressee" even though I have one in the "to" box.  I've tried it a dozen times and it keeps telling me there is no addressee.  Anybody know what the problem might be?  I've been able to send emails after accessing my work email from other computers but not this one.  Any ideas?  Thanks.