An error occurred while saving the commentAnonymous commented
There is a less drastic work around, you can simply null the proxyaddresses attribute for the mail user. (Set-MailUser -Identity <Identityofmailuser> -EmailAddresses $Null). This is essentially what happens if you create the contact first anyway.
Thanks for the feedback! Can you provide more information about how you prefer guest policies to work and if you think allowing guests the same level of permissions as any other member is desirable? Additionally, would you like to control policies for specific guests or just have blanket policy for all guests? Please keep the feedback and votes coming! Thanks!