I have several domains hosted on the same VPS.(for example, a.com, b.com, c.com)
I've set up postfix+sasl+dkim for sending emails locally with one domain (for example: a.com), and it is working fine.

But now emails sending from other hosted domains will display the real origin is from a.com in the mails header information.

How to make it support a.com, b.com, c.com?

For one domain, the origin is easy:
In the postfix main.cf file I add:
myorigin = $mydomain
(other settings are omitted)

For dkim, I add
Domain example.com
in the /etc/mail/dkim-milter/dkim-filter.conf (other settings are omitted)

And /etc/mail/dkim-milter/keys/keylist
[email protected]:example.com:/etc/mail/dkim-milter/keys/example.com/default #private key
I have no clue for postfix origin, but I guess for dkim, I should change
Domain example.com ====> Domain a.com b.com c.com

[email protected]:a.com:/etc/mail/dkim-milter/keys/a.com/default
[email protected]:b.com:/etc/mail/dkim-milter/keys/b.com/default
[email protected]:c.com:/etc/mail/dkim-milter/keys/c.com/default

Any help is appreciated!