Currently when you can only have a single email service set up per account.

This makes it hard for splitting transactional email addresses from marketing email addresses.

For example, in my case, I have sub-domains for different email purposes using MailGun. What complicates this even further is that if my client decides that they don't want to receive newsletter or marketing material and unsubscribe, they won't receive any Transaction Emails from me anymore.

I do believe that is it best practice to separate your emails between the main domain (for transactional) and the sub-domains (for marketing or newsletters)

Although there are some complicated ways around this, it makes it harder for my clients to achieve this outcome.

I do understand that there are a lot of rules and regulations when it comes to emails regarding the structure and laws to protect people from spam, that I still don't fully understand.

But I would like to suggest the following if it complies:

Multiple email services are added with their own DND settings, or at least 2 Email Services with their own DND settings, from which one can be transactional emails for instance, and another for marketing emails.

If I client does not want to receive marketing emails I should still be able to send them, for example, an invoice, because the email service being used for transactional emails does not have DND active. And then if they were subscribed to a mailing list but do not want the marketing emails anymore and unsubscribe, the DND settings are active on the email services being used for marketing, so I can then still send them, for example, an invoice because my main transactional email services DND settings are not active.