Microsoft has confirmed that some Outlook.com users are experiencing issues with emails being blocked and marked as spam when trying to email Gmail accounts.

This known issue only impacts users with Outlook.com country domains, according to a support document published by Redmond on Tuesday.

Affected Outlook users are being told in follow-up emails from Gmail's servers that their messages were suspicious and have been stopped from reaching the recipient's inbox.

"Remote server returned message detected as spam [..]. Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked," the replies from Gmail's mail server (mx.google.com) explain.

Google's support website says that it's very likely that only a subset of these messages are being blocked because they have "a strong likelihood of being spam."

Temporary fix available

Until the Outlook.com Team can address this newly acknowledged issue, Microsoft advises impacted users to add an Outlook.com alias to their accounts and email Gmail contacts using that alias.

Outlook aliases are additional email addresses associated with your Outlook.com account, and they use the same inbox, contact list, and account settings as your primary email address.

Users can sign in to their Outlook.com accounts with any alias because they all use the same credentials.

To add a fresh alias, click "Add an alias" and log in to your Microsoft account if required. Within the "Add an Alias" section, create a new Outlook.com email address and designate it as an alias or add an existing email address as an alias, then click "Add Alias."

After creating the new Outlook.com alias, follow these instructions on how to send email from a different email address or change your primary alias.

Google's new anti-spam guidelines

While Microsoft didn't share the root cause behind Outlook.com emails getting tagged and blocked as spam, Google suggests Redmond should review its new bulk sender guidelines to improve Outlook email deliverability to Gmail accounts.

Google is now blocking emails sent by bulk senders who don't follow stricter spam thresholds and authenticate their messages as required by new guidelines designed to strengthen defenses against spam and phishing attacks.

"Starting in April 2024, we'll begin rejecting non-compliant traffic. Rejection will be gradual and will impact non-compliant traffic only," the company says in a recently updated support document.

"We strongly recommend senders use the temporary failure enforcement period to make any changes required to become compliant."

In June, Google will start enforcing these new requirements to block spam, phishing attempts, and malware from reaching its users' inboxes.