# SMTP Error 550 5.1.1: User Unknown — Causes, Common Messages, and How to Fix It ### Understanding SMTP Error 550 5.1.1 This error is the digital equivalent of a **“Return to Sender”** notification. Basically, an email failed after it was sent over [SMTP](https://www.warmy.io/blog/what-is-smtp-and-how-does-the-smtp-server-work) (Simple Mail Transfer Protocol) and the receiving server did not accept it. **A returned email or SMTP Error 550 5.1.1 is an indication that the address does not exist on the server of our recipients.** Possible reasons include the email address was entered incorrectly, or the recipient does not have an associated account anymore, or there are other issues with the recipient’s mail server. The error message may look something like this: > `550 5.1.1 ... User unknown` Other variations may include “Recipient address rejected” or “Mailbox unavailable.” ## Common causes of SMTP Error 550 5.1.1 SMTP Error 550 5.1.1 can occur due to various reasons—each of which indicates a fundamental problem in the way the email was addressed or routed. ### Invalid recipient email address When you make a mistake typing in the email address and that email address doesn’t exist, the email will not be sent and you will receive the error message. ### Problems with the domain name If the domain portion of the email address (the section after the @ symbol) is incorrect or no longer exists, emails will not be delivered. This can happen when a recipient’s domain name changes or expires. ### Problems with server configuration On the recipient’s side, the mail server may be configured in such a way that it does not recognize or accept emails to a valid address. ### Outdated DNS records As DNS converts domain names to IP addresses. And if the DNS records of the email recipient’s domain are outdated or incorrectly configured, the email servers cannot find the right direction to deliver the email, and delivery fails. ## Step-by-step solutions to resolve SMTP Error 550 5.1.1 Regardless of which email provider you use—be it Gmail, Outlook, Yahoo or another digital email service—let’s look at the general steps: ### 1. Check your email address carefully Make sure there are no typos or errors. It’s easy to misspell a name or misspell a domain, so take another look. ### 2. Check your email settings Double-check your account settings to see if there are any filters or rules that may be redirecting your emails or blocking them entirely. ### 3. Check your server settings If you are using an email client (such as Outlook or an email application), make sure your SMTP server settings are correct. Here are the SMTP server settings for Gmail, Outlook, and Yahoo: #### Gmail SMTP settings - **Server address**: smtp.gmail.com - **Port**: 587 (for TLS/STARTTLS) or 465 (for SSL) - **Authentication required?**: Yes - **Username**: Your full Gmail email address (including @gmail.com) - **Password**: Your Gmail password (or an App password if two-factor authentication is enabled) #### Outlook SMTP settings - **Server address**: smtp-mail.outlook.com - **Port**: 587 - **Encryption method**: STARTTLS - **Authentication required?**: Yes - **Username**: Your full Outlook email address (including @outlook.com, @live.com, etc.) - **Password**: Your Outlook password #### Yahoo SMTP settings - **Server address**: smtp.mail.yahoo.com - **Port**: 587 - **Encryption method**: STARTTLS - **Authentication required?**: Yes - **Username**: Your full Yahoo email address (including @yahoo.com) - **Password**: Your Yahoo password ### 4. Check your spam filters Sometimes outgoing emails may be incorrectly flagged by your account’s spam filters. Make sure your email doesn’t fall into these filters by adjusting them if necessary. ### 5. Be aware of restrictions Please be aware of the sending limits set by your email provider. If you send too many emails you can quickly reach your limit. Here’s a brief overview of the sending limits for Gmail, Outlook, and Yahoo: #### Gmail - **Daily limit**: Gmail allows you to send up to 500 emails per day for personal accounts. For G Suite (now Google Workspace) users, this limit increases to 2,000 emails per day. - **Recipients limit**: Each email can be sent to a maximum of 500 recipients in total for personal accounts and 2,000 for Google Workspace accounts. #### Outlook (Microsoft 365/Office 365) - **Daily limit**: Outlook.com (for personal use) has a daily limit of 300 emails. - **Recipients limit**: The maximum number of recipients per message is 500 for Outlook.com. #### Yahoo Mail - **Daily limit**: Yahoo Mail limits users to sending 500 emails per day. - **Recipients limit**: Each email can have a maximum of 100 recipients. ### 6. Contact customer service If you have fixed everything, but still receive an error, write to technical support. ## List of common SMTP Error 550 5.1.1 messages Here is a list of all the errors you may receive: 01. **550 5.1.1 – Recipient rejected** 02. **550 5.1.1 – Recipient address rejected** 03. **550 5.1.1 – Recipient address rejected: User unknown in virtual alias table** 04. **550 5.1.1 – User unknown** 05. **550 5.1.1 – Mailbox not found** 06. **550 5.1.1 – Invalid mailbox** 07. **550 5.1.1 – Mailbox unavailable** 08. **550 5.1.1 – Unrouteable address** 09. **550 5.1.1 – Mailbox temporarily disabled** 10. **550 5.1.1 – No such user here** ## Fix SMTP errors and employ Warmy.io to make the most of your email strategy In wrapping up, tackling the SMTP 550 5.1.1 isn’t as daunting as it seems. By carefully checking email addresses for typos, ensuring your email settings are spot on, and gradually building your sender’s reputation with services like Warmy.io, you can significantly reduce the chances of hitting this error. Remember, it’s all about taking it step by step and keeping an eye on the details. With these strategies in your toolkit, you’re well on your way to ensuring your emails land right where they should— in the inbox, error-free.