As of late There have been many instances of the following errors:
451 Domain of sender address [EMAIL PROTECTED] does not resolve // the actual domain has been changed 450 Unknown host/domain problemdomain
4xx means, probably, the destination MX had a DNS timeout when querying for problem domain. It happens. be patient. you retry and it DNS answers for them next time.
And bet your company on reliable, timely mail delivery. :))
Len
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