We are seeing elevated rates of hard bounces at Gmail domains starting 14 Dec 2020 at approximately 3 pm US Pacific time. This is not a Marketo-specific issue, but is occurring globally for Gmail for all senders. It seems to have mostly cleared within an hour or so, but we are seeing intermittent issues ongoing since. The error we are seeing most commonly is "(550-5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces." We understand the error is largely erroneous and intermittent.
If you have seen an unexpected hard bounces that meet this criteria since 3 pm US PT 14 Dec 2020, you may revert the invalid=true to invalid=false and retry those that failed. Customer guidance on how to update the impacted leads is here.
More information on the outage reported here: https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=109f5878e60fdfb3c0f64fc9b81752e1 and here: https://www.spamresource.com/2020/12/gmail-bounce-issues-overnight.html.
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