Is there a way to determine if your audience is using Gmail vs. outlook (other then the obvious @gmail.com addresses)? For instance, if you wanted to send different emails to Gsuite shops vs. Microsoft shops?
Marketo doesn't drill down to the individual email client level, but you can source this through email testing/analytics tools like Litmus. You can add a unique identifier to Litmus analytics code and then associate that data back to Marketo once it's been gathered through a few sends.
It wouldn't cover your whole database perfectly as it would require email opens to pull the data, but could be used as an incremental improvement where client is known.
I'm curious as to what sort of differences you'd have across the emails between different clients?
Apart from Litmus, you can also use Email-on-Acid which is cheaper than Litmus.
In one of our campaigns, we included add to calendar links for Google, iOS, and Outlook. Depending on which calendar link did the user click on, we were able to get some insights. In our case, 60% of our audience reads their emails in Google while remaining 40% would use Outlook or other email clients.