Outlook Express response: 550 5.7.1 Authentication Required

Using Outlook Express from a laptop via DSL, outgoing mail is now stopped with the following pop-up window Command sent was: "MAIL FROM < my email address>  Response received : 550 5.7.1 Authentication Required".

Email in Outlook has been verified as to the proper POP3 settings, etc.  Can send ok via Hotmail.  Also sending ok via another computer on the network which uses Outlook Express.  

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Your SMTP server settings for the POP account is wrong!
Check with your ISP for the right settings...
as mattisflones indicated check that your SMTP server settings are correct (You may need to be using your ISP's settings if you're using a domain setting)

Alternatly, if the server you are sending through is using a form of SMTP_AUTH try checking your mail before sending a message
(delete messages in your outbox first )
There's a couple of possibilites here:

1) The SMTP server that you have configured isn't the mail server of your DSL provider. Many mail servers are configured for SMTP AUTH so that users outside of the mail server's local network can relay through the mail server (provided that the sucessfully authenticate) without exposing the mail server to abuse by spammers.

2) You are using your DSL provider's mail server and it has SMTP AUTH enabled and you haven't configured Outlook to authenticate to the SMTP server.

The short answer is in you outlook express account properties select my server requires authentication (Server Tab)  that should solve your problem.

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