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At our office others can use the .NET SMTP client to send email via our Google Apps account. Using the exact same settings and credentials this does not work on a particular machine. It seems to be just this one machine with this issue. I get:

Mailbox unavailable. The server response was: 5.7.1 Invalid credentials for relay

We are all on the same external IP, so it's not

Daniel Williams
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1 Answers1

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Aha! We found the smoking gun. Here is what the message actually says:

SmtpException: Mailbox unavailable. The server response was: 5.7.1 Invalid credentials for relay [ffff:fff:ffff:ffff:ffff:ffff:ffff:ffff]

I've obfuscated the last part, but note that the IP address this appears to come from is an IPV6 address. Out relay whitelist only includes IPV4 addresses. So I turned IPV6 off on that machine - because honestly, what is that good for anyway?

Daniel Williams
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