lspt11 I have the same issue with layouts being blown away when forwarding a newsletter sent via Acymailing and that behavior is happening in both v5 and v6. I think I found what's causing it but I have to verify. If you created your own template and use custom CSS, I think Acymailing does not retain or apply the CSS formatting when the newsletter is forwarded. Or it could be an issue with email client.
Alexandre, how does Acymailing apply Custom CSS styling via selector (class) by the way? Does it convert it to inline CSS when the newsletter is sent or it just include the custom CSS in the header? I noticed that in any email clients, inline CSS seems to be working much better than loading the formatting layout via the header and using selectors.