Our servers are configured to require a password to receive email, but also send email, as this cuts down on spammers attempting to use the server for illicit purposes.

As a result, you will need to configure your email programs to initiate SMTP AUTH - which has the email client send the login and password to the server prior to sending/transmitting email.

There are other helpdesk articles to assist with setting up each type of client to use SMTP AUTH.