Some advice for you.
RFC821 is obsolete, superseded by RFC2821.
1. On the subject of EHLO (which RFC2821 says is preferred, though HELO is still acceptable), RFC2821 says:
"The SMTP client MUST, if possible, ensure that the domain parameter to the EHLO command is a valid principal host name [my emphasis] (not a CNAME or MX name) for its host. If this is not possible (e.g., when the client's address is dynamically assigned and the client does not have an obvious name), an address literal SHOULD be substituted for the domain name and supplemental information provided that will assist in identifying the client."
That is, a well behaved MTA connecting to another to send mail should say:
- EHLO mail.example.com
- EHLO example.com
so please reconsider.
2. You are concerned that when your host says EHLO, it will do so with something that does not match the DNS name of your cable/DSL IP address?
The answer is that ideally these should match though actually a huge proportion do not. You are more likely to run into spam blocking issues as a result of your IP address itself (is it in a dynamically assigned pool, or does it have rDNS of something.dhcp.example.com) than you are because EHLO does not match the rDNS name.
Some sites now block (or at least tag) email that comes from hosts that do not EHLO with a fully qualified host name, so changing EHLO to example.com may be counterproductive.
- if your IP is cable/DSL, relay outbound email via your ISP's smarthosts
- if not, get correct forward and reverse DNS set up (mail.example.com <--> 192.168.0.1)
- and have your Domino host EHLO with mail.example.com