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This is driving me nuts. I have Postfix/Dovecot set up on Centos 6 VPS for my emails. I can send and receive emails without any problems on my outlook (home broadband), on my Android phone (3G connection), however when I set up exactly the same email account on Outlook at my friends house I'm getting bouncebacks when I'm trying to send email (receiving works fine):

554 5.7.1 : Relay access denied

Now if it wasn't working at all I'd understand, but why is it working on several internet connections, but not everywhere? Can someone shed some light on this? Where should I look for the problem?

EDIT: It appears that Outlook is causing the problem, because I've tried Thunderbird on the same PC and it seems to work.

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This is likely because you are setting mynetworks and sending from one of those networks when at home or on your phone (which I assume is using your home network). You need to set up user authentication so you can send from any network. – Patrick Jun 3 '12 at 18:05
The phone is using 3G - mobile internet, this is just to emphasize that I can send email from several different IPs (networks), but not all, for some reason. User authentication is set up and working. – Caballero Jun 3 '12 at 18:31

Is smpt auth configured correctly on the new client computer? My guess is that since everything works on your other machines you've simply forgotten to configure it within Outlook.

If this isn't the case you should try to send an email from a computer that works, look at the logs, send from the non working machine, and then compare for any differences in the logs.

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