Why am I receiving the error 550 Access denied - Invalid HELO name?

All of our servers have a requirement for SMTP authentication in order to send email.
When you make an email account on CPanel, you can click on configure with client and it will list the smtp details for you.

When sending email using an email application on your computer such as Outlook, Thunderbird or Mac Mail and you receive the error "550 Access denied - Invalid HELO name", it means you have not ticked the option "My outgoing server (SMTP) requires authentication" in your email appliation. For steps on enabling this feature in common email applications, please see below:

Microsoft Outlook
From the Menu Bar, Click Tools then Click E-mail Accounts
Select "View or change existing e-mail accounts" then Click "Next"
Select your Email account then Click "Change"
Click "More settings"
Select "Outgoing Server" tab
Select the box beside "My outgoing server (SMTP) requires authentication"
Select "Use same settings as my incoming email server"
Click "Ok"
Click "Next"
Click "Finish"
Thunderbird
From the Menu Bar, Click Tools, then Account Settings
This will bring up the Account Settings window where you can click on Outgoing Server (SMTP) at the bottom of the list in on the left side of the window.
Back in the middle of the window, select the appropriate outgoing server and click the "Edit" button.
Make sure "Authentication method" is set to "Normal password"
Click OK to close the SMTP edit window and again on the Account Settings window.
Mac Mail
From within Mac Mail Click Mail, Preferences, then Accounts
Near the bottom of the window, look for Outgoing Mail Server (SMTP), click the drop down arrow and select "Edit SMTP Server List..."
Click the Advanced button in the middle of the window and make sure Authentication is set to "Password."
Click OK to close the edit window
Microsoft Outlook
  1. From the Menu Bar, Click Tools then Click E-mail Accounts
  2. Select "View or change existing e-mail accounts" then Click "Next"
  3. Select your Email account and click "Change"
  4. Click "More settings"
  5. Select the "Outgoing Server" tab
  6. Select the box beside "My outgoing server (SMTP) requires authentication"
  7. Select "Use same settings as my incoming email server"
  8. Click Ok, Next and then Finish.
Thunderbird
  1. From the Menu Bar click Tools, then Account Settings
  2. This will bring up the Account Settings window where you can click on Outgoing Server (SMTP) at the bottom of the list in on the left side of the window.
  3. Now select the  appropriate outgoing server (ours) and click on Edit.
  4. Make sure "Authentication method" is set to "Normal password"
  5. Click OK to close the SMTP edit window and again on the Account Settings window.'
Mac Mail
  1. Within your application click Mail, Preferences, and then Accounts
  2. Where it says Outgoing Mail Server (SMTP), click the drop down arrow and select "Edit SMTP Server List..."
  3. Click the Advanced button and make sure Authentication is set to "Password."
  4. Click OK to close the edit window
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