The URLs deliberately expire for security reasons.
I’d suggest you develop a process to actually save the HTML (and subresources like images) offline.
I know some of the workarounds are sending a copy to a mailbox and storing there, but that's a bit clunky.
Sending to a mailbox is a pretty common tactic for compliance/archiving apps. But the catch w/Marketo is you either have to
(a) remember to include the archive mailbox in every relevant send or
(b) use the automatic CC feature, which requires major work from your IT team (i.e. to discard duplicate emails if you’re not actually trying to store a separate copy for each recipient as one would in a regulated industry)
Another option is to build an integration that regularly scans the activity log using the REST API, then downloads the HTML for every unique email ID (not unique person!) that goes out. You’d end up with a much neater system. But o’course you need a sharp developer to build such a thing.
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