Re-run the setup to rebuild customers files...
This system isn't intended to be used that way, if you add it only like that, you will only create the "panel" side email, but the system itself will not know them because they were NEVER created (you by-pass the base process of i-MSCP adding system, which run some backend command to add it to the system (Dovecot/postfix/courier/...)).
If you don't wan't to have any trouble, add them manually on the panel