Welcome, if you would like to post a comment please register.
We also encourage you to explore all things Zimbra with our team and members of the community.
Short question: is it at all possible to differentiate on max message size between domains? Say one domain should be allowed 10MB, whilst another might be set to 50MB.
I don't think you can configure that through the Admin GUI but there should be no reason why you could not create a Postfix service policy to do it. PolicyD is able to handle per domain max message sizes.