Using whatever version of MS Outlook (within an MS Exchange corporate network), I keep receiving a "non existing recipient" error when I send mails to existing recipients.

For ie, I send a mail to Jack Sauber, he receives it, but I receive a Sysadmin notice warning me

that "one recipient could not receive your mail: George McCabe is a non existing recipient".

First thing I did is to check if there's a rule that links any user I am sending mails to .. to the non existing recipients: no, there's no rule..no aliases..no relation at all between the existing recipients and the non existing ones. And I am not even picking my recipients from any mailing list or whatever mail groupings where the non existing users could have been members of: I am writing manually the mail address of the user directly in the SEND TO: field.

Since the message came from the sysadmin, I did contact them, and they say that there's, on their part,

nothing wrong (no rules, no redirections, no aliases..nothing!).

What could it be (I am sure that it's at the sysadmin level ...but I guess they just dont know what to look for)

Thanks

It's look like a redirection problem. You could also check that this recipient didn't make a rule to forward mails to another person.