Sorry I copied that from a reasonably decent looking site I found on Google and didn't look at the quotes. May have also been my iPad screwing it up. This is an entry direct from one of my SMTP servers (although I've changed the name):
Code:
define(`confDOMAIN_NAME', `smtp-inbound.mydomain.com')
In my case I have it near the bottom of the mc file, just above the MAILER lines. As I said you shouldn't really need this as Sendmail will just use the server hostname by default, I just find it strange that you say your server is coming back with
otherdomain.com, which is why I suggested just hard coding it. I only have this set manually because I have a few load balanced mail servers with different hostnames, and
smtp-inbound.mydomain.com is the name associated with the IP address we use for inbound email on our load balancer. Not that I actually use Sendmail now, I've actually moved this and most other servers to Postfix.
1) my host shows the reverse DNS set properly for the IP addresses yet drill xxx.xxx.xxx.xxx does not return the domain name but I'm going to bed.
drill ip.address will not look up reverse DNS. You're better off using
host ip.address.
What do you mean your host shows it set properly? I assume you're aware that reverse DNS entries will almost certainly need to be added for you by the ISP that provide the IP address? (unless they have some control panel where you can create reverse DNS entries).
2) Can I get a HELO for the other domain names on the same server with different (or same) IP addresses?
That's not really feasible** and you don't want that. Mail servers are not any different to any other server on the Internet. It has a hostname, one hostname, such as
mail.myisp.net or
mail-east1.us.some-other-crap.myisp.net, really doesn't matter. All that matters is that the hostname resolves to the correct IP address, and the reverse DNS entry for the IP address maps back to the same hostname.
If you want to accept email for 3rd party domains all you need to do is the following -
1) Make sure your mail server accepts the emails, which in Sendmail is done by added the domain to
/etc/mail/local-host-names.
2) Get the inbound emails for the domain delivered to your server's IP address. You can either set the MX on their domain to
my-mail-server-hostname.mydomain.com, or you can create an A record for
mail.customerdomain.com that points to your server and use that in the MX.
Other servers couldn't care less that your server doesn't announce itself as mail.customerdomain.com when they receive or send email to it. It's perfectly normal for server.someisp.net to handle email for somecustomer.com. All they care about is that your server announces a valid hostname (which Sendmail normally just picks up from the system hostname) and that the forward and reverse DNS match up (which ideally should be the case for any server on the Internet). For a start an IP address can only logically resolve back to one hostname. **You'd have to use a different IP address for every customer domain, and the only obvious way I can see to make that work in Sendmail is to actually give each customer their own instance of Sendmail running in jails, which may be nice but a waste of time, effort & resources.
They know customers will just complain to littleisp.com if their emails end up in Junk, rather than complain to Google about perfectly genuine emails being marked as spam. That's a lot better for them than relaxing their filters and having the possibility of users complaining to them because something that is spam has managed to make it into the Inbox. Welcome to email in 2017. 
)