How does zimbra use DNS, Split DNS, Local IP

[zimbraxtc]

Hi all Zimbras!

Im going to install a 8.8 on a Ubuntu 18.

First google-hit was this tutorial:
https://computingforgeeks.com/how-to-in ... 18-04-lts/

States I need a DNS-server.

Checking docs and searching forum:
https://wiki.zimbra.com/wiki/Split_DNS

Saying: "Depending on how the firewall and network are configured, the external address may not even be reachable from the mail host, and mail will not be delivered. "

In my case I have a FW and external ip and a local-ip for the mailserver.

My external ip-address for the server are accessable by the server.

I dont care about performance since it is just a handfull of domains.

Question 1: can I install WITHOUT a split-dns, and use my external DNS?
Question 2: is the issue with dns-record a "binary"-problem or can it work sometimes and sometimes not?

Thanks a lot in advance.