If you are using Outlook Express, Outlook, or Windows Mail, A HELO error ( 550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1) can sometimes be generated by leaving the "My server requires authentication" checkbox unchecked. You may need to try checking off this checkbox.
Use the below examples to find this checkbox:
1. Outlook 2000
Go to Tools > Services (or Tools > Accounts, if Services is not listed). Select your Internet E-Mail Account and press “Properties.” Go to the “Servers” tab, and check “My server requires authentication” under Outgoing Mail Server. Press “Settings,” select “Use same settings as my incoming mail server,” then press “OK” three times to save the changes & return to Outlook. Finally, restart your computer.
2. Outlook 2002 & 2003
Go to Tools > E-Mail Accounts. Select “View or change existing e-mail accounts” and press “Next.” Select your Intermedia.NET POP/SMTP account and press “Change.” Press “More Settings.” Go to the “Outgoing Server” tab, and check “My outgoing server (SMTP) requires authentication.” Select “Use same settings as my incoming mail server” and press “OK.” Press “Next,” then press “Finish” to confirm your changes and return to Outlook. Finally, restart your computer.
3. Outlook Express
Go to Tools > Accounts. Select the “Mail” tab. Select your Intermedia.NET account and press “Properties.” Go to the “Servers” tab, and check “My server requires authentication” under Outgoing Mail Server. Press “Settings,” select “Use same settings as my incoming mail server,” then press “OK” twice times to save the changes, then press “Close” to return to Outlook Express. Finally, restart your computer.