Error 554 Relay access denied

Hello all,
Suddenly have a problem sending emails via Outlook ExpressV6 in XP Pro. I can not send emails. Whenever i try to send, it shows following message:
"Error 554 Relay access denied"
Can anyone fix this please?

Thank you in advance.

I think the person sending mail has incorrectly configured their settings for SMTP authentication. When sending email via TextDrive servers to addresses outside of your hosting server, you must provide SMTP Authentication via your email client. Make sure your username and password is provided in your SMTP configuration and you will be allowed to relay mail.

A host can require that their SMTP be used to send outgoing mail for domains other than the host domain, or that the exact email address or login be used for outgoing emails. That's why it is important to consult with the host or ISP. The email client has nothing to do with this error. Since the host is rejecting the emails, this is a host issue. Try to delete and recreate the email account.

This error message is similar to a Recipient Address rejected, in that the server reporting the error does not like the domain name; however, in this case, the problem is that the outgoing email server did not like the domain of the sender's email address (as opposed to the above Recipient errors, where the incoming mail server does not like the recipient's domain).

What this error generally indicates is that the outgoing server you're using (usually the one assigned to you by your ISP) will not accept email with the From: address you've used on the email. For example, domains such as BellAtlantic.com, GTE.net, and Verizon.com do not allow users to send email their outgoing mail server if the "From" email address used on the message is not the one assigned by them (e.g. you can't send out mail that uses your Pobox address as the From: header via their mail servers).

Many ISPs have begun to enforce these restrictions to prevent spammers from using the ISP's access and SMTP server to send spam. To remedy this error, you will need to use the email address assigned to you by the provider as the From: address on outgoing email.

This may be due to the use of outgoing server (SMTP) on field outside provider. In order to avoid the problems of SPAM, SMTP servers access providers are not configured to work only for their subscribers.
You just have to configure your mail client (Outlook) to use the outgoing mail server of your company or your ISP.