Check your /etc/mail/sendmail.cf file for the following:

# who I send unqualified names to (null means deliver locally)
DR

# who I masquerade as (null for no masquerading) (see also $=M)
DM

If there are no entries here, it may be that the user you attempted to send to is not in the /etc/passwd file and it did not know where to send this to thus it gives an error. Since you are sending to your domain (someuser@my_company.com) and if those fields are blank, then it did not know where to send the email to. So it checks /etc/passwd for the username. If it's not there, it can then check /etc/mail/aliases for an alias for the user name

somuser: someuser@otherserver.my_company.com

If there isn't an alias, it won't send.

At some point there has to be a mail server that knows about all the users. My environment had an Exchange server so all email not specifically for the internet (someone@not_my_domain.com) or not specifically for a local user or aliases ( allunixadmins: joe, tom, charlie, scott) would go to the Exchange server. If your Sendmail server will be the main mail hub, then all users must be listed in /etc/passwd or have an alias set up in /etc/mail/aliases.

You need to check out the FAQ at Sendmail and/or read the Sendmail book to figure out how you want to move mail around. Once you figure that out, more specific help can be given.