Domain Name on Email sent from iseries

Hi
When the iSeries sends email, using SDNSMTPEMM command, how does it determine the domain part of the sender's email?
Our email is being sent out with a from address like this:
userprofile@xxxxxxxxxxxxxxxxxxx

It should look like this:
userprofile@xxxxxxxxxxx

Why is the "iseries" part being added to the domain?

If we add the user profile to the directory (WRKDIRE) and specify the SMTP address (F19=Change name for SMTP) for the user, the email from address comes comes from there and appears as we want it to appear.

If we don't add the user profile to the directory, email can still be sent, but the from address is, "userprofile.iseries.company.com"

Do we have to add each user to the directory? Then specify the SMTP user id and SMTP domain for each user?

We're trying to avoid having to add directory entries and SMTP user ids for hundreds of users.

Here are the 4 commands we used to configure email on the the iSeries:
CHGTCPDMN HOSTNAME('iseries') DMNNAME('company.com')
CHGTCPHTE INTNETADR('10.555.55.55') HOSTNAME((company.com) (iseries))
CHGSMTPA RTYDAY(0 0) RTYHOUR(0 0) MAILROUTER('EXCHANGE.COM') FIREWALL(*YES) DIALSCD(*NO)
ADDDIRE USRID(INTERNET SMTPRTE) USRD('SMTP email') SYSNAME(INTERNET) PREFADR(*NETUSRID *IBM ATCONTXT)


Thank you