I am able to receive e-mails, but I receive an error message on sending?
Some ISPs, such as Pandora, block the use of SMTP servers not their own. This is to prevent the sending of spam via their servers. If you set up the SMTP server (e.g. mail-out.pandora.be for Pandora) of your ISP as outgoing mail server (outgoing mail server), you will be able to send out e-mail normally.