Why is Outlook not sending the emails straight away?

Hi @Joseph WAN,

Good day! Thank you for reaching out to the Microsoft Q&A forum.

There are several possible causes that could be affecting Outlook not sending emails straight away, or delays in email delivery to Gmail recipients.

Here are some possible causes of your issue:

PART 1: Emails from Outlook (O365) to Gmail are delayed

1. Outbound Mail Flow Delays from Microsoft 365

• Microsoft 365 may occasionally queue messages if Gmail's mail servers are busy, throttling, or applying greylisting.
• This is usually shown in message trace logs with delays at the Remote Server stage.

2. Recipient Server Delays (Gmail)

• Gmail may defer or delay delivery if:
  • Your email is flagged as suspicious or spam.
  • Your domain or IP is missing SPF/DKIM/DMARC records.
  • There's a misconfiguration causing Gmail to distrust your sender.
• Throttling or Rate Limits: If your domain has sent a large volume of mail to Gmail recently, Gmail may rate-limit you temporarily.
• Outlook in Cached Mode: Outlook might appear to send mail, but delays can happen if it's offline or in cached mode, especially with a large outbox or unstable connection.
• Antivirus/Firewall or Add-Ins
  • Security software can delay email delivery if it's scanning outgoing messages.
  • Problematic Outlook add-ins might also interfere.

PART 2: Emails from Gmail to Outlook (Inbound delays)

If you're referring to emails from Gmail arriving slowly in Outlook, then consider:

1. Microsoft Exchange Online Protection (EOP) filtering

• Inbound emails may be delayed due to spam/malware scanning or transport rules.
• Message trace in the Microsoft 365 admin center can confirm this.

2. Transport rules or connectors

• Custom connectors or rules may delay Gmail messages.

3. Third-Party security gateways

• If your org uses services like Mimecast, Proofpoint, etc., those can delay delivery.

Here're some suggestions you can try:

1. Run a message trace (_requirement admin role)

• Go to Microsoft 365 Admin Center > Mail flow > Message trace
• Trace a delayed email to see where the delay happened:
  • Was it stuck in Microsoft 365?
  • Was it delayed by the Gmail side?

2. Check DNS settings (SPF, DKIM, DMARC)

Make sure your sending domain has corrected DNS records:

SPF: v=spf1 include:spf.protection.outlook.com -all
DKIM: Enabled in Microsoft 365 (admin center)
DMARC: v=DMARC1; p=none/quarantine/reject; rua=mailto:dmarc@example.com

3. Try OWA (Outlook Web App)

• Send the same email from Outlook on the Web.
• If it's instant there, but delayed in desktop Outlook, the problem is likely client-side (Outbox stuck, network, add-in, etc.).

4. Review local Outlook settings

• Go to Send/Receive tab (on the top menu ribbon)> Send/Receive Groups > Define Send/Receive Groups
• Check "Schedule an automatic send/receive every X minutes" (typically 5 or 10 minutes) and "Perform an automatic send/receive when exiting"
• Also check File > Options > Advanced > Send and Receive settings.
• Make sure it’s set to "Send immediately when connected".

I hope this information helps address your concerns. If you need any further assistance or clarification, feel free to reach out. We're here to help. Thank you very much for your understanding and your cooperation.     

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