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When using Lusha Engage with a Microsoft Outlook mailbox, you may encounter a 550 5.1.8 Access Denied, Bad Outbound Sender error. This occurs because Microsoft’s email servers have flagged your email address as potentially compromised or as a source of spam, leading to undelivered emails.
The issue typically happens due to company inbox limits.

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How to identify the problem

  • You will receive a Non-Delivery Report (NDR) email in your mailbox from Microsoft. This email contains details about the failed delivery and instructions to contact your administrator.
  • You will also see a top banner in Lusha Engage with the note: “Some emails failed to be delivered due to company inbox limits. Contact your system administrator to fix it, then reactivate the relevant sequences.”
  • A Failed status will be visible on affected sequences.
Failed status banner on a sequence due to Outlook sending error

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What happens when emails fail

When Lusha Engage detects an Outlook issue, the non-delivered email is marked as Failed. All scheduled emails are cancelled by pausing active sequences, and contacts’ sequence delivery is paused.

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How to resolve this issue

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1. Contact your email administrator

Reach out to your organisation’s email administrator to address the issue. Unblock the user’s account. If the account is secure, the administrator can remove the user from the restricted table by:
1

Log into Microsoft 365 Defender

Logging into the Microsoft 365 Defender portal.
2

Open Restricted Entities

Navigating to Email & Collaboration > Review > Restricted Entities.
3

Unblock the affected user

Selecting the affected user and clicking Unblock.
4

Confirm the action

Confirming the action.
Unblocking may take up to an hour to complete.

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2. Review your anti-spam protection settings

Discuss with your system administrator to review your sending limits and, if needed, request to increase them. Here is where the administrator can configure anti-spam protection settings in the Microsoft 365 Admin Center:
1

Open the Admin Center

Go to the Admin Center.
2

Open Anti-Spam protection settings

Navigate to Security > Email & Collaboration > Policies & Rules > Threat Policies > Policies > Anti-Spam > Protection Settings. Recommended settings:
  • Restrict sending to external recipients (per hour): Set to 6.
  • Maximum recipient limit per day: Set to 50.

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3. Reactivate your Lusha Engage sequences

Before reactivating, ensure you have reviewed your internal email limits with your system administrator and received their approval. Once confirmed:
1

Log back into Lusha Engage

Log back into Lusha Engage.
2

Reactivate the failed sequence

Reactivate the sequence flagged as Failed by switching the toggle on, or clicking the 3 dots of the sequence and pressing Activate in the action menu.
Reactivating a sequence flagged as Failed
3

Emails resume automatically

The emails that weren’t sent will be rescheduled from the same point they stopped.

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Preventing future issues

Monitor sending limits Enhance account security
  • Enable multi-factor authentication (MFA).
  • Use strong, unique passwords.
Update security protocols
  • Regularly update SPF, DKIM, and DMARC settings to ensure email authentication.
If you have any questions, feel free to reach out to the support team via live chat from the Lusha website, your Lusha Dashboard, or via email at support@lusha.com.