When you send an email, the server displays the error:
Server: 'mail.domainname.com', Protocol: SMTP, Server response: '550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1)', Port: 25, Security (SSL): No, Server error: 550, Error number: 0x800CCC78
This error usually occurs because you have not enabled authentication for your account in the email program.
Check in your email program settings to see if you have the option checked:
For example in Outlook, in Tools - Accounts - Preferences - Servers - "my outgoing server (SMTP) requires authentication", select this option and choose the "use same settings as incoming mail" option.
For other clients like Thunderbird, etc. The configuration and steps are different but all email clients have this option and it must be checked.
