Is your Outbox growing faster than your Inbox? Do your friends despair? Does Outlook Express make you despair with error messages like "Message could not be opened from Outbox folder." or "Some errors occurred while processing the requested tasks."? Does Outlook Express send multiple copies of outgoing mail?
Many a configuration misstep (say a port change enforced by your email provider but not really communicated to you) and Outlook Express problem (such as a corrupt Outbox folder) can halt your outgoing mail. Fortunately, there's more than one thing you can try to remedy the situation — and send mail again.

Troubleshoot Outlook Express Mail Sending Problems

To make Outlook Express send mail again:

Check your outgoing mail server settings.

Select Tools | Accounts... from the menu, highlight the desired account and click Properties.
Make sure the correct server name is entered under Outgoing mail (SMTP): on the Servers tab.
On the same tab, verify My server requires authentication is checked if needed (which is typically the case). Under Settings..., you can specify a user name and password different from your incoming mail credentials.
On the Advanced tab, make sure This server requires a secure connection (SSL) is checked under Outgoing mail (SMTP): if your outgoing mail connection must be encrypted.

Check the port under Outgoing mail (SMTP):, too. Typical ports include "25" and "465".

Make sure your Sent Items folder is not overly large.
The folder can hold 2 GB at most. To check the size, go to your Outlook Express store folder and examine the Sent Items.dbx file's size.

Move messages from the Sent Items folder in Outlook Express to another folder. Set up folders for all mail sent in a year, for example.

Make sure you compact folders manually after moving messages.
Rename a corrupt Outbox.dbx file.

With Outlook Express closed, open your Outlook Express store folder in Windows Explorer and rename the Outbox.dbx file to "Outbox.old".

Note that you can no longer access any messages in your "old" Outbox folder.
If the renaming has fixed your delivery problems, you can delete the Outbox.old file.
If nothing helps, you can have Outlook Express create an SMTP log file to find out more about what's going on behind the scenes.