Exchange 2016 and Sophos Configuration (DNS Settings?)

Hello,

OK, I'm scratching my head hard. I know I am probably missing something trivial, but for the love of God I can't figure it out:

I have a lab here with DC01 and EXCH01 VMs. I attached the DNS config from Godaddy.

(for the sake of security, I removed the official domain I am using, let's assume it's lab.com)

Mailflow inside my network is working. Outside not - but not failing completely... let me explain:

Tx/Rx Outlook16 on my local machine = OK

Tx/Rx OWA = OK

Tx/Rx iPhone in WLAN = OK

iPhone on LTE = no autodiscover, no manual config, no Rx

mxtoolbox.com returns all green on MX

I can ping mail.lab.com, smtp.lab.com and autodiscover.lab.com.

I have two zones in my local DNS, lab.com and internal.lab.com. Lab.com is hosting two entries, autodiscover and mail, both pointing to the IP-Address of the Exchange Server.

In the Sophos:

Internal (Network) -> Email Messaging -> Any

DNAT Internet IPv4 -> SMTP -> Ext. WAN Address, Translate to EXCH01

That's about it.

Any ideas?

Thank you