There are several options in the app center. There’s Kopano, OX App Suite, EGroupware, and NextCloud. NextCloud is actually what we are using for webmail and calendar mostly because we already used it for file access. It’s included mail app is fairly new and probably not what I would pick if you were going to use it as your main email interface, but it works well enough for occasional access if you need to get to email from a browser. We mainly use our phones if we are not at a computer where we can use the standard desktop tools. We did use Horde for a time, but the calendar access from the phone and desktop apps didn’t quite work right for us. NextCloud’s calendaring seemed to work a bit better for our needs.