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?
Please move the question to Products (Move link at the right). This isn't a support space.
And include images inline, not as attachments. Not all Community users can see attachments, which makes them very confusing.
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