Have someone who gets this error message whenever they put more than two email address in the To: field. Even just two addresses causes the error. Only one of the addresses receives the email.
The error message starts with "Your message did not reach some or all of the intended recipients." Then goes on to say..."The following recipient(s) cannot be reached:" and lists them. Finally ends with the error in the Title.
In the past they have used distribution lists a lot. But this error is not the result of using a list, just normal sending.
However the error doesn't occur when an email with two addresses in the To: field is composed from their email provider's webmail site. Neither does it occur if their email account is setup in Outlook on another computer used in another location.
Their internet connection at their house from where they are sending has been checked and is fine.
Anyone got any ideas?