How to Fix "550 Access Denied - Invalid HELO Name" Error:
For Microsoft Outlook:
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Open Outlook.
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Go to File → Account Settings (or Tools → Email Accounts in older versions).
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Select your email account and click Change.
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Click More Settings.
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Go to the Outgoing Server tab.
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Check "My outgoing server (SMTP) requires authentication".
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Select "Use same settings as my incoming mail server".
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Click OK → Next → Finish and restart Outlook.
For Thunderbird:
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Open Thunderbird.
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Go to Tools → Account Settings.
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Scroll down and click Outgoing Server (SMTP).
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Select your SMTP server and click Edit.
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Ensure "Authentication method" is set to "Normal password".
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Click OK, then close settings and restart Thunderbird.
For Mac Mail (Apple Mail):
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Open Mail and go to Mail → Preferences → Accounts.
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Select your account and go to Server Settings (or SMTP section).
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Click Outgoing Mail Server (SMTP) dropdown → Edit SMTP Server List.
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Select your server and click Advanced.
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Ensure Authentication is set to "Password" and enter your credentials.
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Click OK and restart Mail.
Additional Checks:
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Ensure your username and password are correct.
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Verify that your SMTP port is correct (usually 587 with TLS or 465 with SSL).
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If using a custom HELO/EHLO name, ensure it matches your domain (some servers block invalid HELO names).
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If the issue persists, contact your email provider for the correct SMTP settings.
