I have updated one of our client contact email address - firstname.lastname@example.org to receive communication. However, when i look a the activities log it gives the attached error. I have communicated to this client via this email on Outlook so it's an active address. Can you please confirm why this might be occurring?
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This is just an answer following reasoning.
The emails that Marketo sends are different to what you send out of Outlook. And I don't mean graphically. The security systems that accept or block incoming email care about what's happened in the backend.
Marketo emails are sent via the Marketo email servers, which has a different IP to the Outlook email you sent. Since Marketo emails are "list" emails they'll have different email headers, and a few other characteristics.
Things to check are:
DKIM and SPF are set up correctly in Marketo
Your Marketo sending IP isn't temporarily blacklisted