Trouble receive mails from 1 specific domain

Hi all

Hope you can point me in the right direction here.

We have an ASG220 running as plain firewall.

We are getting mails delivered from an external scanner agency, that takes care of scanning for spam, virus etc.

We are somehov having trouble getting mail from one specific domain, and the external agency tells us that we should disable SPI.

The reason for this should be that it somehow disturbs the delivery, because it is dropping the packages at a very low level.

When they contant the mailserver, the receive a reponse like this: 220 ************************************************* from the firewall.

What does this mean - and does anyone else have any experience with this?

The mailserver is behind the firewall, and we have a standard DNAT rule mapping the SMTP traffic from the external agency to our mail server.

Have tried different types of NAT, but they all yield the same result: 220**********************

How can I set up the Astaro so that the reponse comes directly from the mailserver?
If possible.......

Best regards.