Since Munchtions are getting discontinued, I had to work on alternative ways lately. In that process, built a set of webhooks. See if these can help you solve similar situations.
Angus, apparently I spoke too soon and this feature is not generally available yet. There is only a very limited-scale test right now. I believe there are plans for a larger scale test (like a beta test), but it may take a little while.
I too tried this once by using the field "domain name". I just add domain names to a set of emails and used that domain field in smartlist to filter out the email address for the corresponding domains. But marketo does the filter only to email address not to domains... also please tell me if there is any way on this.
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