You must successfully login to a POP account (any POP account on your server) no more than 30 minutes before sending mail. You can configure your email client to check for new mail every 30 minutes to maintain continuous access, or simply make sure to check mail before sending it on an intermittent connection.
Some ISPs (such as AOL and Earthlink) have blocked outgoing SMTP (TCP port 25) traffic in an attempt to prevent spam from originating from their networks. One sign that this may be occurring is that you can receive but cannot send mail through the same server.
If your ISP is blocking outgoing SMTP traffic, you must configure your email client to send mail through your ISP's SMTP server (typically, "smtp.isp.net"). POP username, password, and other settings should remain unchanged. Please consult your ISP account literature or support department for the correct setting.