[stratosmacker]

Sorry for naivety, what is HELO in this context? I've been running a mail server with the other aforementioned things set correctly and tested; I've been able to reliably deliver mail to all of the big senders for years now, albeit on a small scale.

Am I missing something not knowing what HELO is ??

[account42]

HELO (or more usually EHLO) is the initial greeting of SMTP protocol which includes the mail server's address.

AFAIK plenty ofservers will outright reject all mail if you only have an IP address and not a domain name there.