Gmail users Not Receiving my Outlook.com emails any longer.

For Outlook.com, the most relevant causes are recipient-side spam filtering, temporary server issues, account sending restrictions, and message content/signature issues.

Try these checks on the Outlook.com side:

1. Confirm the message is not over the send limit conditions
   • If the Outlook.com account is newly created, it can have a temporary low sending quota until credibility is established.
2. Remove anything in the message that may trigger filtering
   • If the message has a signature, remove the signature and resend a test message.
   • If the message has an attachment larger than 25 MB, Outlook.com cannot send it.
3. Improve recipient acceptance
   • Ask affected Gmail recipients to add the Outlook.com address to their safe senders list.
   • Add the recipients to Contacts in Outlook.com.
4. Check whether this is a temporary delivery condition
   • Some delivery problems such as connection timeout or remote server connection issues can resolve on their own. If no non-delivery report is returned, wait a short time and resend a test message.
5. If Gmail is blocking the message as spam or low reputation
   • Outlook.com guidance for blocked/listed errors is that the recipient’s email provider support team must unblock the account or domain.
   • A known Outlook.com issue affecting some country domains sending to Gmail was marked fixed.
6. If Outlook.com account validation is involved
   • Outlook.com has a documented workaround for send issues caused by Microsoft account validation problems: follow the Microsoft account security info and verification steps from the Outlook.com known issues page.

If the problem is only affecting some Gmail recipients and there is no bounce message, the supported Outlook.com actions are to simplify the message, remove the signature, avoid large attachments, add recipients to Contacts, and have recipients safelist the address.

References:

• Can't send or receive email in Outlook.com | Microsoft Support
• Sent email in Outlook.com comes back "delivery failed" | Microsoft Support
• Fixes or workarounds for recent issues on Outlook.com | Microsoft Support