Found this at GOOGLE
Tom
I'm using Outlook with SBC DSL. Quite often my emails do not go through. An undeliverable notice enters the inbox immediately with comment
"553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1) Seems weird, since the domain is Yahoo.com. I've contacted the SBC tech people and everything "checks out" from their end and they had no clue what this comment meant. Receiving email is OK.
Help, please.

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