I m getting this error message when saving Office 365 Mail Flow new Rule.
There are multiple recipients matching the identity "xxxxx@domain.com". Please specify a unique value.
I use Office 365 Mail flow transport rule to Redirect Message to many users, some are internal domain users, and some are external mail contacts. It's the mail contacts ( external users) that are causing the error message, no issue with internal domain user. For the mail contacts, I also took the extra step by adding them as 'New Guest Users' in Azure Active Directory Users through invite.
Strange, if I add the mail contact to a distribution group, I don't get any error prompting multiple recipients matching the identity. But if I add the same mail contact to a group of users under Mail Flow transport rule 'Redirect message to' group , I get the 'multiple recipients matching the identity' error message.
In an experiment, I deleted all the Guest Users from Azure Active Directory ( those users with #EXT# in their email addresses), I could now add the mail contacts to the Mail Flow transport rule 'Redirect message to' group, but then emails now failed to deliver to the mail contacts that are in the distribution groups. Any help is appreciated. ERAH