This happened to me a little while ago after a re install and was because one of the identities was playing up. Strange thing was that we could send from both identies. Go to File>manage identities, delete(or remove, not sure which) each identity and then add each identity again......I found that deleting the identities one at a time did not work. All the identities had to be cleared first.
The only remaining niggle is that it will remember the password for one identity but not for the other but I can live with that.
My isp is NTL but it is probably the same for others.