I'm posting this because google and microsoft search turned up empty.

We have an exchange server for our email set up, however it is reporting 5.1.1 errors for email adresses that we KNOW to be valid...this ofcourse bother us a lot, but we cannot find a culprit for it.

Has anybody had similar issues or does anybody know what direction to search in, if you need more information please ask, i am unsure what i should provide you with in this case.