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Hi Matt Moore,
The 550 message is an email bounce. The "oob" stands for "out of bounds". There is a previous thread about this with links and more in-depth explanations. What does "550 internal oob The recipient is invalid" mean ? ...... Hard Bounce / Soft Bounce ?
This could mean a variety of things, though most likely that the email address, for one reason or another, is invalid. The thread here gives some good details:
Specifically, look at Mike Reynolds reply at the bottom of the thread. It gives some good clarity into the "[oob]" part of that error code - basically, the email server said it would accept the email, but then bounced it. (so don't count on the "550 [internal] [oob] The recipient is invalid." error to mean "the email isn't valid" - this is possible, but not the only option.)