XG 115 preventing incoming traffic in bridge mode

I hope I've just missed something small...

I've installed an XG115 to filter e-mail. It's gateway is a Sonicwall TZ-210.

The XG115 has been configured as a bridge with email protection as per these documents:

www.sophos.com/.../Deploy-in-Pure-Bridge-Mode.pdf

https://community.sophos.com/kb/en-us/122973

https://www.sophos.com/en-us/medialibrary/PDFs/documentation/SophosFirewall/Pocket-Guides/ProtectInternalEmailServerMTAMode.pdf?la=en

Workstations and the XG115 can reach the gateway and internet hosts.

However the gateway cannot ping the XG115 or anything on the other side of it. Thus preventing our services from being reached and e-mail from coming in.

Thoughts?

T.