There are 2 options to solve this:
1.- Set mailNickName as "first.last" on the AD user objects:
Configure CMN DC to set attribute mailNickName as "first.last". But would verify first that there are no existing users already with that same name and surname. Also, it is a bit tricky to change the mailNickName to users that have already being synced to O365, since UPN (User Principle Name) will have been already set based on the mailNickName attribute and changing it might be challenging.
2.- Set Notes Users Forwarding address as "email@example.com":
Export the Notes Directory from MNE to a TSV (Tab Separated Value) file, and add corresponding ‘firstname.lastname@example.org’ addresses in the ‘TargetForwardingAddress’ column, then import the TSV file. Finally when you launch the manage mail routing operation, you can select that column to update the Forwarding Address field in the Notes User affected. So you will not need to set the Notes User Document Forwarding Address manually.