On Tue, 8 Oct 2019 00:25:44 -0500 David Wright <deb...@lionunicorn.co.uk> wrote:
> > > > "subscribed.address" is the HELO and can be what I want it to be. > > See the headers of my previous mail. > > Why would you use a "subscribed.address" (presumably an email address) > for your HELO (presumably actually a EHLO). I was under the impression > that it should be a domain, ie a FQDN. It should actually be a hostname, it's the official public name of the mail server, though not of course its local network name. It must be resolvable in public DNS. In practice, not much of this is enforced, and you can get away with a domain name and, apparently, an email address. It's a while since I did any email diagnostics, but certainly the HELO pretty much only had to exist, it didn't seem to be checked very much. I have seen email from a Small Business Server refused because the default HELO was the invalid domain name xxxxx.local, as someone had forgotten to set it properly. I can't be bothered (i.e. I haven't had to do it yet) setting different HELOs for each domain that I use, and it has never been a problem. Neither does the MX record have to match any email address, nor the PTR record for the sending IP address. There are many complex setups where a business might send through one third-party SMTP server and receive through the SMTP server of a mail-cleaning service, for example. The only constraint (again, so far) on a sending address PTR is that it must have a complementary A record, which does not have to be the address that the MX points to or anything related to the email itself. Many ISPs cannot handle multiple PTRs for the same IP address, at least not in their user control panels, though they are permitted by RFC. -- Joe
