The Nextcloud setup guide does not set any email address for the admin user created by occ maintenance:install.
This leads to an issue in the mail setup as sending a test mail does not work because no email address is set for the admin user. See also nextcloud/server#56609
Additionally the mail address of the admin account is also used by Nextcloud for administration related notifications, so that it might be a good idea to set it.

The default value could be [email protected].

Setting this can be done with php occ user:setting $NEXTCLOUD_ADMIN_USER settings email "[email protected]" according to https://docs.nextcloud.com/server/stable/admin_manual/occ_command.html#user-setting