Emma is not currently set up to support DMARC for customers. Using a domain with DMARC implemented will result in message failures for any receiving domains that check for DMARC records (so, most of them). The reason we can't support DMARC is that we do not support using DKIM keys specific to each customer. We don't have a good way to store those keys currently and the methodology for storing and using them doesn't exist in the app ... and it might not for some time.
That being said, there are two options our delivery team recommends:
You can stop using DMARC. This works best in cases where you haven't implemented DMARC consciously, but maybe did it just because it popped up as an option. It's less compelling when your security team sees it as a necessary measure to secure email communications.
You could add "sp=none" to their DMARC record, then start using a subdomain when sending from Emma (i.e. -- emma.domain.com). The "sp" part says that any subdomains of the domain do not use the top level DMARC settings, but rather have their own that take precedence. You could create a DMARC record for the subdomain emma.domain.com with "p=none" instructing receiving domains to take NO action should DMARC authentication not align and this would effectively end the bounce issue.
Please let me know if you have any additional questions here.