If you're not accessing your mail box from that ISP's network, they usually block "sends" for security reasons (i.e., you can receive mail, but not send it).
Some ISPs will offer an option for mail accessable from any net, but the mailbox is "outside" the ISP's protection. Some ISP's will offer a "webmail" option, but restrict use to text and only a few kinds of attachments.
FWIW
Scott