How to set up sendmail to forward all mail to an external MTA

For the delivery part of email destined for one of these users you can use the "virtusertable" feature. To redirect the email of only some users to an external address, e.g. for your sample users you could add this to your sendmail.mc file:

FEATURE(virtusertable, `hash -o /etc/mail/virtusertable')

Then in the /etc/mail/virtusertable file use something like this to deliver email for a@abc.com locally, and email for b@abc.com or @c@abc.com* to the remote, hosted server:

a@abc.com localuser

b@abc.com b@hosted.server

c@abc.com somebody@hosted.server

Rebuild the virtusertable.db map by running makemap(1), and Sendmail should be able to handle the delivery part of email ok:

# cd /etc/mail

# makemap hash virtusertable < virtusertable

Controlling which local user is allowed to send email is slightly more involved. You might be able to use "relay_mail_from" and access map entries, but since I haven't actually tried this, I'll let someone else answer with more useful details.

As you noted, you need a DNS wildcard.

When mail from the wildcard addresses comes in, sendmail doesn't know what to do with it. You can fix that by adding a sendmail rewrite rule to rewrite the wild subdomain parts into the main domain part.

in your sendmail.mc:

LOCAL_NET_CONFIG  
R $+ < @ $+ .example.com. > $*    $: $1 < @ example.com > $3       dnl

Note that you need a tab to separate the left-hand side rule from the right-hand side replacement. ($* ends the left-hand side; $: starts the right-hand side.) Note that the trailing dot on the left-hand side may or may not be necessary. Also note you need a second tab after the right-hand side and the comment (between $3 and dnl).

You'll want to make sure example.com is in your relay-domains file.

Once the domain has been collapsed by the rewrite rule, you can route all mail for the domain to a single user with a virtusertable entry:

@example.com  luser

That should about do it.