You'll know if you are dealing with a Relay Denied error because it will look like the error message below:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was '<YOUR THIRD-PARTY EMAIL ADDRESS>'. Subject 'test8', Account: 'a.mail.sonomait.com', Server: 'mail.sonomait.com', Protocol: SMTP, Server Response: '554 <YOUR THIRD-PARTY EMAIL ADDRESS>: Relay access denied', Port: 25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC79

What this error message means, was that you tried to send email through our servers from a third-party email address such as Hotmail, Yahoo, your work email address, etc. You can only send emails through our mail servers using your Zimbra hosted email account.

To fix this, you will want to configure your email client to authenticate when it contacts our SMTP server. This will enable you to send all email through our servers, because your email client will be identifying itself using your Zimbra hosted email account login info. 

Mozilla Thunderbird

1. Open Thunderbird
2. Go to Tools > Account Settings
3. Select "Outgoing Server (SMTP)"
4. Click on the "Edit" button
5. In the "Security and Authentication" section check the box for "Use name and password"
6. Type in your Zimbra hosted email address in the "User Name:" box.
7. Click on the "Ok" buttons to apply the changes

Outlook Express

1. Open Outlook Express
2. Go to Tools > Accounts
3. Click on the "Mail" tab
4. Select your email account and click on the "Properties" button
5. Click on the "Servers" tab and check the box at the bottom, in the "Outgoing Mail Server" section, next to "My server requires authentication". Then click on the "Settings" button.
6. Mark the bubble next to "Log on using", then enter in your Zimbra hosted email account login information.
7. Check the box for "Log on using Secure Password Authentication". Click "Ok"
8. Click on the "Apply" button, then click "Ok" to close that window.

Outlook

1. Open Outlook
2. Go to Tools > E-mail Accounts
3. Select the "View or change existing e-mail accounts" and then click on the "Next" button
4. Select your email account from the list and then click on the "Change" button
5. Check the box that says "Log on using Secure Password Authentication (SPA)", then click on the "More settings" button
6. Select the "Outgoing Server" tab and check the box that says "My outgoing server (SMTP) requires authentication"
7. Select the bubble for "Log on using" and enter in the login information for your Zimbra hosted email account. Then click the "Ok" button
8. At the next window, click on the "Next" button, then click on the "Finish" button.