This is not in the improvements category. Nextcloud is locked to save on resources and when you unlock it you will be on your own, on each update you will receive a warning. However, it does work and is possible to add an email plugin, please read in Unsupported Modifications. Remember all these plugins after unlock, decrease system resources.
I know that the default webmail (Roundcube) might not look so artsy but it is much more stable than the rest.