# How to Resolve Outlook 5.1.8 Sending Error

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.

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💡 The issue typically happens due to company inbox limits.

## 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.


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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.

## How to resolve this issue

### 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. Logging into the **Microsoft 365 Defender** portal.
2. Navigating to **Email & Collaboration > Review > Restricted Entities**.
3. Selecting the affected user and clicking **Unblock**.
4. Confirming the action.


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💡 Note: Unblocking may take up to an hour to complete.

### 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. Go to the **Admin Center**.
2. 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**.


### 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](https://dashboard.lusha.com/engage).
2. 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.


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


## Preventing future issues

**Monitor sending limits**

- Follow [Lusha's best practices for email limits](/user-guide/performance-and-troubleshooting/email-limits-best-practices).
- Stay within Microsoft's sending limits to avoid restrictions.


**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.


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💡 If you have any questions, feel free to reach out to the support team via live chat from the [Lusha website](http://www.lusha.com/), your [Lusha Dashboard](http://dashboard.lusha.com/dashboard), or via email at [support@lusha.com](mailto:support@lusha.com).