553 validating sender

Spammers use the relaying ability of SMTP servers to mask the true origin of junk email by relaying it through third-party servers that permit such open relays.

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The SMTP server must connect to another SMTP server to relay the message.

When you send an email message that encounters a relay error, your SMTP (outgoing) email server might return your email message with an error message such as one of the following: The exact error message might vary, depending on your Internet service provider (ISP).

The same situation could occur when you're away from your office and try to send an email message using your work email account.

Relaying occurs when an email message is sent to an email address whose domain (the name after the @ symbol, such as adatum.com) is not processed by the Simple Mail Transfer Protocol (SMTP) or outgoing server that the sender is requesting to deliver the message.

Being that only one client has this problem, I would say the setting is correct.