If your Outlook Express was working fine and suddendly causes error when trying to send email, then your Microsoft Windows may have done some updates to your system.
There has been several reports, that checkmark required for "My server requires authentication" in Outlook Express gets removed after Windows update.
The usual symptoms are able to receive emails but errors when trying to send out emails. You will need to put the checkmark back in to "My server requires authentication" in your Outlook Express setting. It's in TOOLS->ACCOUNTS->PROPERTIES->SERVERS. Please visit our Help Desk for photo diagram for detail help.