Postfix Configuration problem (domain of sender address does not exist)

In main.cf:

# SENDING MAIL
# 
# The myorigin parameter specifies the domain that locally-posted
# mail appears to come from. The default is to append $myhostname,
# which is fine for small sites.  If you run a domain with multiple
# machines, you should (1) change this to $mydomain and (2) set up
# a domain-wide alias database that aliases each user to
# [email protected].
#
# For the sake of consistency between sender and recipient addresses,
# myorigin also specifies the default domain name that is appended
# to recipient addresses that have no @domain part.
#
#myorigin = $mydomain
myorigin = $myhostname

This is the default setup. If your hostname is domain.local this is probably why you are seeing that as sender domain.

You need to fix your domain name in the postfix configuration (domain.local is not a valid domain name).

Check /etc/postfix/main.cf, there are a few ways this can be specified, this may help: http://www.postfix.org/BASIC_CONFIGURATION_README.html#myorigin

This is one of those issues that you will need to involve/bribe/blackmail your DNS admin for assistance on as you will want the name your server claims when trying to send to be correct. Mnay servers will bounce your mail if the sender doesn't correctly resolve both forward and reverse.

For example, let's say your server is mail01.example.com. When your server, with the $myorigin correctly configured to advertise as mail01.example.com, contacts my server, my server will do a DNS lookup of of the IP address you're connecting from and will expect the lookup to match the name your giving me. If it does not correctly resolve, then I will sever the connection because you're probably just sending me spam.