Outlook has started throttling (not sending) emails going to other 365 accounts.

Hi Gale,

Thank you for reaching out and sharing the details of your issue.

Since you already have a support ticket open with Microsoft, it’s recommended to continue working through that channel. Please keep the assigned support engineer updated that the issue is still ongoing and that emails are not being sent successfully, so they can provide more targeted troubleshooting steps for your tenant.

Please note that we don’t have access to your tenant or the ability to check detailed diagnostics on our end. For in-depth investigation and backend verification, it’s best to continue working with Microsoft Online Support, as they have the necessary tools and visibility to review your environment directly.

In the meantime, you may want to review the following areas:

1. Outbound spam protection / sender restrictions

Microsoft 365 may automatically throttle or block outbound emails if it detects unusual sending patterns, compromised accounts, or spam-like activity. In such cases, messages may be delayed, routed differently, or users may be restricted from sending.

Reference: Outbound spam protection - Microsoft Defender for Office 365 | Microsoft Learn

2/ Tenant outbound recipient limits

Exchange Online enforces tenant-level limits on external recipients. If your organization exceeds these limits, additional outbound emails may be delayed or blocked until usage falls back within the allowed threshold.

3/ Service-side mail flow issues

Large mail queues can occur if Exchange Online cannot deliver emails and continues retrying. This can be reviewed using queue and reporting tools in the Exchange Admin Center.

Reference: Queued messages report in the new EAC in Exchange Online | Microsoft Learn

Additionally, you may ask your IT Director to investigate the following:

Go to Exchange Admin Center > Mail Flow > Message Trace:

• Review a sample of queued messages and check detailed delivery status, including any SMTP or NDR error codes.
• Reference: Message trace in the new EAC in Exchange Online | Microsoft Learn

In the Microsoft Defender portal, review alerts such as:

• “User restricted from sending mail”
• “Suspicious outbound email activity”
• “Email sending limit exceeded”

Go to Exchange Admin Center > Reports > Mail Flow

• Check the Tenant Outbound External Recipients Rate report to confirm if limits have been reached.

If Microsoft Support has already provided the steps above and the issue still persists, you may ask them to further investigate the following:

• Mail flow rules (transport rules): Verify if any rules were recently added or modified that could be rejecting, redirecting, or altering messages.
• DNS and email authentication: Confirm that MX, SPF, DKIM, and DMARC configurations are correct and properly aligned.
• Accepted domain configuration: Ensure domains are correctly set as Authoritative or Internal relay, especially in hybrid environments.
• Mail source (users vs. apps/devices): Identify whether the queued emails are coming from user mailboxes, applications, or devices, and validate the method used (SMTP AUTH, relay, Direct Send, etc.).
• SMTP AUTH settings: Check whether SMTP AUTH is disabled at tenant or mailbox level, or restricted by security policies.
• Remote domain and forwarding settings: Review configurations that might affect auto-forwards, auto-replies, or system-generated emails.

I hope this helps you move forward with the investigation. If you need any clarification, please feel free to reach out. I’d be happy to assist further.

Thank you for your patience and understanding.

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