If we have a member of the database who has the email address email@example.com, is there any way to find out whether the email client is outlook, gmail, yahoo or something else?
The reason we want to know is so that we can decide which email clients to prioritise when it comes to our email design testing.
Any info much appreciated.
No... and yes.
It's more complex than just "the email client." Most people in your database are going to be using at least 2 devices. And the email provider (Gmail, Yahoo, Outlook 365, 3rd-party hosted service, on-premises Exchange or IMAP infrastructure) isn't the same as the email client.
For example, the Gmail web app, Gmail mobile app, built-in OS mail app, and/or 3rd-party mail client can all read Gmail-hosted mail. For another example, one of my accounts is an Exchange 2016 mailbox, but I only read it using Outlook Web Access or a non-Microsoft Android app, never via Outlook (desktop) nor the Outlook mobile app.
You can see the User-Agent that downloaded the Marketo tracking pixel by looking at the Browser details of an Opened Email activity. Some User-Agents allow you to fingerprint the email client and/or provider. For example, a request from "GoogleImageProxy" -- unless forged for some reason -- has been forwarded from the Gmail web client. (Of course checking the Opened Email details only works for people that have images enabled. If I don't have images enabled and I click a Marketo-tracked link, you won't know where I viewed the body of the email.)
3rd parties do allow you to embed additional tracking pixels in Marketo emails to automatically get deeper diagnostics like those mentioned above. Also, checking the MX records for a domain can give you insight from the other side: if someone is using Corporate Google-hosted mail, their MX records will include aspmx.l.google.com, so you'll know at least that much even before sending to them.