Why can I receive, but sometimes can't send E-mail?
In some cases, port 25 which is used for the SMTP connection (outgoing mail) is blocked by the ISP or by a firewall. This sometimes is noticed when using your laptop on a public network or other location away from home.
Of course, you can just use webmail access to your E-mail account to get around this issue.
A more permanent solution is to set up your E-mail client (Outlook, Thunderbird, etc.) to use port 587 and SMTP authentication. The specific steps vary depending on which client you're using, but are generally as follows: