Prevent mail to Gmail users from being blocked or sent to spam

Prevent mail to Gmail users from being blocked or sent to spam

https://support.google.com/mail/answer/81126

External email accounts can not send emails to Exchange Server group email addresses

This is a security policy. Open Exchange Management Console. Go to Recipient Configuration | Distribution Group, right-click a group name. Click Properties | Mail Flow Settings | Message Delivering Restrictions. The check box Require that all senders are authenticated restricts external accounts from sending emails to this group.