Does Marketo have any documentation on ways that users can improve email delvivrability? The only product documentation I can find is the page about setting up SPF and DKIM. I am wondering if there is any other documentation here.
This is a good place to start: Email Deliverability Cheatsheet. Also, just search the community https://nation.marketo.com/search.jspa?q=deliverability and you will find a number of good resources.
Sub-domain and Email Setup in Marketo
Marketo Email Reputation Management
Also, you need to setup list automation:
I cannot tell you what a difference this will make within 2-6 weeks. I've helped firms go from 87 or less to 99%, although it's common to go from 92-98% usually.
Keep in mind that INBOX placement will not necessarily improve because of this. But it may over time.
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