Confused about ZCS attributes

[saf]

Hi,

I'm trying to change the outgoing host name (more details below), but I'm confused about which ZCS attribute should I use. Maybe zimbraSmtpHostname? I wouldn't want to change zimbraServiceHostname, as it seems a bit risky.
More details about my setup. It's actually nothing special, but I will give as many details as possible:
The ZCS servers sits in a DMZ, has a private IP (192.168.0.5) and is setup with an internal hostname (mail-01.domain.internal). It is NATed and has port forwards to a public IP (198.51.100.5), on which the reverse DNS is set to match the public MX name (mail.example.com). The public MX name also matches the published DNS(DKIM/SPF)/CN entries.
In effect it's:
192.168.0.5(mail-01.domain.internal)->198.51.100.5(mail.example.com)->outbound
When sending an email (which almost always works just fine), I will usually get a T_DKIM_INVALID, because the ZCS identifies itself as mail-01.domain.internal.

So (finally): what attribute should I change to convince my server to identify itself as the public name, instead of the "actual" internal name when sending emails?

Thanks.

Ubuntu 16.04.3 LTS
ZCS 8.7.6.GA.1776.UBUNTU16.64 UBUNTU16_64 FOSS edition.