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I have Google Apps setup for my business and use Gmail as my mail host. I can send email just fine, but there are a couple addresses to which I cannot send e-mail. I get the error

550 550 Authentication is required for relay (state 13)

I have MX records setup as well as the SPF record that Google suggests. Outside of that, I am stuck. Any ideas? The domain is synasync.com

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migrated from superuser.com Aug 22 '11 at 19:56

This question came from our site for computer enthusiasts and power users.

Sounds like you are reaching an SMTP-MSA server instead of an SMTP-IN server. Possibly the MX records for the recipient domain are misconfigured.

Does this only happen when you try to send emails to one specific domain? Do you see the error if you send from a non-Gmail account?

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Sorry Sarah - not sure how I missed your response to this. It only happens on a couple of domains and pretty rarely. I see this error from my Google Apps account to domain A, but I can send from my regular Gmail account to domain A just fine. – skaz Sep 10 '11 at 15:23

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