This is not an Entourage or OutlookExpress error
Are the POP and SMTP settings exactly the same that the ones in Maill.app ?
This an Apple Unix system error:
Error -3253 is "Connection reset by peer", that means "tried to send the message, but the server refused the connection".
(1) try to use SMTP settings different from the one the Web provider you're using allows you.
Supposes you're web provider is, let's say Comcast, and you want to send messages through the Yahoo.com SMTP server. Comcast may prevent you for doing this, to avoid spam relay.
So in you SMTP settings put Comcast's settings, not Yahoo ones, even if the POP settings are the Yahoo ones, the mailbox of which you want to retrieve
(2) an attachment exceeding the limit your email provider allows.
(3) On macosx, Entourage V.x lead to problem due to mac firewall enabled, especially if sending a email with an attachment.