"550 5.7.1 Could not find a valid Account ID (0) for the given domain(domain.com) (id: X0xxxx)"

There may be a couple situations you might see this:

• The sender is using an email address with a domain that is either not subscribed on our system or not aliased to a subscription.  In this case our system will reject the message.  To solve this the sender will need to change their sender address to use a domain that is setup on our system.
• Or, you might be forwarding a message out via our outbound system.  If this is the case, your email server may be using the original sender's address (such as @gmail.com), so our system blocks the message (since the sending domain is not subscribed or aliased on our system).  There are a couple steps to solve this:
• Ensure you have the Smart host setup option enabled in the Outbound > Outbound settings page.
   
• Once that is enabled, our system will allow you to forward emails on two conditions:
• The envelope sender uses a domain that is subscribed/aliased, and the From address uses the original sender
• The envelope sender is blank, but the From address uses a subscribed/aliased domain.

To change the sender address of a message that is being forwarded, you might be able to accomplish this by using Sender Rewrite Scheme (SRS).  Here is our Postfix article about this:

Allow Postfix to Forward through Outbound Service

We do not have an article that explains how to use Sender Rewrite Scheme (SRS) in Exchange or Office 365 yet, but here's a Google search that may help:

Setting up SRS on Exchange (Google search)