The non-delivery report, or (NDR) for short, which states 550 5.4.1 recipient address rejected: access denied, is generated by email providers to inform you that the email you sent was not delivered to the end recipient. The "550" error code is a standard SMTP response code indicating a permanent failure, and "5.4.1" typically signifies a specific type of failure related to recipient address problems and access denial.
The exact cause for this non-delivery (and solution) will vary based on your individual situation. Microsoft advises on their documentation that this non-delivery report means the recipient's email server is not accepting emails from your overall sending domain (@example.com), and that this is usually due to server or DNS misconfiguration. However, you’ll regularly see people posting issues stating they’re having this issue while both the recipient and sending server have correct configuration.
Therefore, in this post, we’ll outline the most common reasons for this issue with email sending, including the solutions, workarounds, and best practices to avoid and resolve this problem, in these three categories:
Sender issues causing 5.4.1 rejections
These are specifically the issues on the side of the email sender that could lead to your email being blocked/rejected by the recipient's server and how you can resolve these potential failure points for email delivery.
Your domain/IP has been spam-blocked:
Email providers want to stop dangerous or unwanted messages from reaching their user's inboxes. There are two primary identifiers they use to track companies and servers' email activity (Domains and IPs).
When either your domain or IP is associated with spam/unwanted emails, as a line of defense, email providers will start to block the delivery of your messages to inboxes, resulting in a bounce. The main ways email providers gather data on unwanted emails are:
- Measuring what number of contacts manually report emails from your domain as spam.
- Analyzing the likeness of your email content to past spam emails.
- Seeing if the activity from your IP address (email server), is frequently associated with spam.
The other step email providers take when they believe your message might be unwanted is to filter it into the spam folder. This won’t result in a bounce, and companies like Allegrow track how often this happens to their customers.
The resolution to this problem: Involves a thorough analysis of your sending practices and measures that can be taken to increase positive engagement and lower spam risk. It’s the hardest cause of these errors to fix and requires ongoing attention to ensure the problem doesn’t worsen. The most common areas of adjustment to improve results and decrease this issue are: Contact selection/risk, Volume adjustments and Content analysis.
Invalid Recipient Address:
If the recipient's email address does not exist on the recipient's email server, the server may reject the email with a "550 5.4.1" error. This is because the server cannot deliver the email to a non-existent mailbox and, as a result, denies access. Typically, you’ll have contacts that don’t exist in your lists and contacts due to the following reasons:
- The email address was guessed and is incorrect.
- The recipient has moved to a new job.
- The recipient has changed email accounts or domains.
The resolution to this problem: is to use a solution that scans your contacts to verify/check them before you send emails to them, like the Email Safety Net as an example. Unfortunately, regardless of where you source contacts from, they go stale at a rate of 30% each year, even for opt-in lists (where people have subscribed and given you their email). So you can expect this to happen even faster to lists that include scraped or guessed emails in bulk.
One thing people don’t realize is that the average rate for normal corporate email users across every industry is 0.48%, which means just under 5 bounces in every 1,000 emails sent. So when outbound email users start to have a rate which is significantly higher than this very small level, email providers take notice and start blocking emails they send to valid addresses too.
Recipient Server Policies:
The server of the recipient you’re reaching out to may have specific policies in place that reject emails from certain senders or domains. One great example of how this can happen is that an email system admin might configure their inbox to only accept emails from senders that have been added to their address book or trusted senders. The systems that cause these can be mainly summarized by the following mechanisms:
- Whitelist Policies. Which can either be by contact or domain.
- Blocking by IP Range. Is less common but can be conducted at the server level if your IP range is specifically associated with spam.
- Email Security Gateways. Barracuda/Proofpoint block emails from domains listed in their internal threat intelligence databases.
The resolution to this problem: Is to ask the recipient to add you to their trusted sender list / address book before emailing them again (if you know them/have a relationship with them). Or on the other hand if you need to outbound email people, you should screen your lists in advance to try and avoid addresses with a propensity to do this to help mitigate outbound risk. While ensuring your organization monitors bounces and domain blacklists closely to address sending issues that are leading to blocking.
Receiver issues causing 550 5.4.1 rejected emails
It’s important to note, while there are more potential reasons for this issue being shown due to an issue with the receiving server – if you’re a sender and have seen this type of NDR multiple times across different organizations recipients. It’s more likely that one of the sender issues applies to you.
One additional potential receiver cause for this, which we won’t expand upon below, is actually an issue occurring with the availability of your email providers service, essentially meaning they’ve had an outage impacting your account. However, this will become obvious if it applies and can be checking the service status of your email provider/tenant. You can do this for Microsoft in the MS 365 admin center as an example.
Incorrect MX record
If external senders receive a "550 5.4.1 Recipient address rejected: Access denied" NDR when sending emails to recipients in your domain, it typically means that their emails are being directed to an invalid or misconfigured mail server. This occurs because the MX record for your domain, which tells other mail servers where to deliver email, is incorrect. As a result, the recipient's mail server cannot find the appropriate destination to deliver the email, leading to the rejection.
To resolve this issue, ensure your MX record points to a valid mail server by checking with your domain registrar/DNS hosting service.
For Exchange Online domains, the MX record should follow the format ‘<domain>.mail.protection.outlook.com’. Additionally, verify that only one MX record is configured for your domain, as Exchange Online does not support multiple MX records.
For Google Workspace, ensure your MX records are correctly set to point to Google’s mail servers. For accounts created before April 2023, use the five standard MX records. For accounts created after April 2023, you can use the single MX record ‘SMTP.GOOGLE.COM’.
You can also test your MX record using the Outbound SMTP Email test in the Microsoft Remote Connectivity Analyzer to ensure it is correctly configured. Or do a free test of your Google Workspace domain in the Google Admin Toolbox (CheckMX).
Domain configuration issues
Admins can identify domain configuration issues causing "550 5.4.1" errors by monitoring for frequent Non-Delivery Reports (NDRs). If external senders report receiving "550 5.4.1" errors when trying to contact your domain, it may signal configuration problems.
To ensure your domain is properly configured for Microsoft 365 users, start by verifying that your domain appears as Active in the Microsoft 365 admin center. Select the domain and use the Troubleshoot option to follow the wizard's steps for resolving any issues. If you manage your DNS records, further check the status in the Exchange admin center by navigating to Admin > Exchange, then clicking on Mail flow > Accepted domains. Ensure your domain is listed with the correct Domain Type value, which should typically be ‘Authoritative’, unless configured as an ‘Internal Relay’ for shared domains.
Directory-Based Edge Blocking issues
Setting up Directory-Based Edge Blocking (DBEB) in Exchange Online specifically controls the reception of emails sent to invalid recipients within your organization. It prevents external senders from sending messages to email addresses that do not exist in your organization’s directory.
DBEB does not control or restrict the sending of emails to external recipients outside your organization. It focuses on blocking inbound emails to non-existent recipients within your organization. Therefore, internal users will still be able to send emails to both internal and external recipients, as DBEB does not affect outgoing email traffic.
Therefore, if you manage the email infrastructure of an organization and have lots of 550 5.4.1 NDRs being raised for senders that attempt to contact people at your organization the likely causes and areas to check for a resolution are as follows:
- Misconfigured Accepted Domains, as issues with this can cause emails intended for valid recipients to be rejected.
- Sync Issues with Azure AD, as DBEB relies on the Azure Active Directory (Azure AD) to determine valid recipients.
- Edge Server Misconfiguration, because edge servers or mail gateways that are not correctly configured to work with DBEB might reject emails for valid recipients.
- Incorrectly Applied Transport Rules as Transport rules in Exchange Online can sometimes cause legitimate emails to be rejected if they are too restrictive or incorrectly configured.
- Improper Licensing or Service Plans, due to certain licensing or service plan issues might affect the functionality of DBEB.
Hybrid deployment configuration issues
Hybrid deployment configuration issues between on-prem Exchange and Exchange Online can cause "550 5.4.1" errors. These errors might occur if the Send connectors and Receive connectors in your on-prem Exchange organization, which are used for hybrid mail flow, are misconfigured. These connectors are typically set up by the Hybrid Configuration Wizard, and if problems arise, the wizard may need to be run again by your Exchange administrator to ensure proper configuration.
For more details on transport routing in hybrid deployments, see the topic "Transport Routing in Exchange Hybrid Deployments.”, Directly from Microsoft support.
Top 5 best practices to avoid bounces
Email bounces are detrimental to your email marketing efforts, affecting both your deliverability and sender reputation. Implementing best practices can significantly reduce bounce rates and ensure your emails reach their intended recipients. Beyond the specific causes and resolutions for 5.4.1 NDRs here’s five general best practices to avoid bounces:
Use non-spammy content
Ensure your email content is relevant, professional, and free from spammy elements such as excessive exclamation points, all caps, or suspicious links. High-quality content that engages your recipients reduces the likelihood of your emails being flagged as spam.
You can monitor engagement benchmarks for outbound content to see how you currently compare to your peers, and even set-up a spam test to see which versions of your content are being filtered the most often.
Throttle your emails
Ensuring you throttle your emails for deliverability will decrease your chances of getting blocked and raising ‘red flags’ with email providers. This process is about picking a sensible email limit depending on the type of sending you’re conducting and gradually increasing sending up to that limit.
Warm up your email domains
Warming up your email domains helps to introduce them to real word-sending environments and get them ready for higher levels of volume. If you’re using a 3rd party SMTP (like an opt-in) newsletter, you can start by referring to Klayviyo’s guidance on getting a domain ready for warming.
Whereas, if the concept of email warm up is completely new, seeing a full guide on how it works here, could help you implement inside any sending solution or goal.
Removed/sunset disengaged recipients
Regularly clean your email list by removing or sunsetting recipients who have not engaged with your emails over a certain period. Keeping your list updated with engaged recipients improves deliverability and reduces bounce rates. As an example, you could implement a sunsetting policy on your contacts where anyone who hasn’t opened an email in the last 60-days is removed.
Ask to be added to the safe sender list
Finally, although this will only work with customers, encourage your recipients to add your email address to their safe sender list. This can help ensure your emails are delivered to their inbox and not their spam or junk folders and send a positive signal to email providers.
