unexpected "sender" address in outgoing e-mail

Hi,

I've installed and configured Postfix 2.6.6 on CentOS 6. I've changed very little from default, this is the diff from the default config:

myhostname = hostx.domainx.ca
mydomain = domainx.ca
myorigin = $mydomain
inet_interfaces = all
mydestination = $mydomain, $myhostname, localhost.$mydomain, localhost
mynetworks = 127.0.0.0/8
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination

I've also added an A record for hostx.domainx.ca, an MX record for hostx.domainx.ca and a PTR record for the host IP that resolves to hostx.domainx.ca.

I can successfully send mail from an external account to osuser@domainx.ca, and it gets delivered to a local mailbox (osuser is a valid user account on hostx.domainx.ca). I can also successfully send mail from osuser@hostx.domainx.ca to an external account. The problem is that delivered e-mail is coming from: osuser@hostx.members.linode.com

myorigin is set to $mydomain, and mydomain is set to domainx.ca, so I expect the mail to come from osuser@domainx.ca.

What am I missing here?

Thank you in advance.