Emils from support center being blocked.

Hi,

At the moment a client is not able to receive any emails from our support centre.
Every email that is sent is being returned with this message:
This is an automatically generated Delivery Status Notification.

Unable to deliver message to the following recipients, due to being unable to connect successfully to the destination mail server.

They are only having this issue with us and our personal email addresses get through.

We have been in contact with them and this is the information they have given us:

For some reason the mails gets to the external mail server but it cannot be passed through the Firewall to the Lotus Notes server.
The IT guy thinks there is something hidden in the mail header of the mail at your end but as it does not come through he cannot track this issue.

If more details are needed let me know.

Thanks,
Conor