I think you may be having the same problem as me. I recently asked here No Nethserver email via Nextcloud Mail app but have had no luck getting it to work.
I haven’t found anyone here who uses the Nextcloud email.
I did post an error from my logs but haven’t been able to work out how to fix it.
[mail] Fatal: Updating account failed: Server denied authentication. PUT /nextcloud/index.php/apps/mail/api/accounts/3 from *[public IP address]* by *[MAC address?]* at 2019-09-17T07:28:12+00:00
Let me know if you get it working!