Our website online forms were previously working, but every so often we have to re-add the SMTP server settings in the Host Settings. Not sure why, but this drops out every few months.

So I have just re-added the "SMTP Server and port", but now getting this error message when clicking "submit" on the website's online forms:

"Mailbox unavailable. The server response was: Access denied - Invalid HELO name (See RFC2821 4.1.1.1)"

I haven't changed any of the settings, so not sure why this is suddenly appearing.

I am needing some step-by-step help with how to fix this, and appreciate any help.

Thanks in advance