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Sometimes in my mail.log I see bounced emails with error messages

550-Please turn on SMTP Authentication in your mail client, or login to the 550-IMAP/POP3 server before sending your message

I tried to find out what the meaning of this error and how I can fix it.

So far with no success.

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up vote 1 down vote accepted

If this is being reoprted by remote MTAs, then there's nothing you can do short of contacting the site admin and telling them to fix their server.

If it's being reported by your MTA, then the next question is how have you configured your mail server? The second question is why haven't you configured you're mail client to reflect the MTA config?

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thank you very much – com May 8 '12 at 10:29

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