Hi Michael,
Your assumption is that the email was not sent out because the email was wrong (I guess you mean badly formed?) - So first I would try to determine if this is really the case. Try doing some tests with correct and incorrect emails. See how they behave and nail the cause.
You you nailed the scenario, then we can try determining the cause.
On top of my head I wouldn't have an answer for you on why this is happening if it is really the case.
Best regards,
Aderson