Email Error Message: "554 5.7.1 Service unavailable; Client host [...] blocked using..."

This article describes an email error with the message "554 5.7.1 Service unavailable," which occurs when the sender's IP address is on a blacklist. This is done to protect servers from spam. The solution is to enable SMTP authentication. Instructions for enabling authentication are provided for various email programs such as Outlook, Mac Mail, Thunderbird, iPhone, and Android. These instructions include detailed steps to ensure the correct settings for the outgoing mail server are used.

Error

554 5.7.1 Service unavailable; Client host [...] blocked using...

Cause

Your IP address appears to be on a blacklist (DNSBL / RBL). To protect servers from abuse by external spammers, it is necessary that only authorized users can send emails. This is ensured by authentication using the mailbox password.

Solution

Please enable SMTP authentication:

Outlook 2013
Outlook 2016/19
Mac Mail
Thunderbird
iPhone
Android

Outlook 2013

1. Access Tools – Account Settings.
2. Select the relevant account and click Change.
3. Click on More Settings.
4. In the Outgoing Server tab, select The outgoing server (SMTP) requires authentication.
5. Make sure Use same settings as my incoming mail server is selected.
   
   
    

Outlook 2016/19

1. Go to File → Account Settings.
2. Then double-click the account you want to change.
3. Click on the menu More Settings → Outgoing Server.
4. Select The outgoing server (SMTP) requires authentication.
5. Make sure Use same settings as my incoming mail server is selected.
   
   
    

Mac Mail

1. Open Mail → Preferences from the menu bar.
2. Navigate to Server Settings for the affected email address.
3. Ensure that the username and password are correctly set in the SMTP settings.
   
   
    

Thunderbird

1. Open Tools → Account Settings.
2. Go to Outgoing Server (SMTP).
3. Select the outgoing server and click Edit.
4. Enable SSL and enter the server name of the email address.
5. Choose Password, normal as the Authentication method and enter your email address as the Username.
   
   
    

iPhone

1. Under Settings, select Passwords & Accounts.
2. Select the relevant email address.
3. Tap on SMTP Settings and then on the Primary Server.
4. Select Password as Authentication and enter your email address as the Username.
   
   
    

Android

1. Open the Email app and go to Settings.
2. Select the relevant account.
3. Scroll down and tap on Outgoing server settings.
4. Enable the option Login required.