Domain – Sending mail with Postfix from multiple domains on the same server

Postfix appends the value of the main.cf 'myorigin' parameter to unqualified sender addresses. So you should set $myorigin to whichever of 'domain1.com', 'domain2.com' or 'domain3.com' is the normal default.

Postfix doesn't do anything special to prevent any address/domain being used as the sender. You can enforce it if you like using a check_sender_access map in smtpd_mumble_restrictions.

If you want to use the value of the 'From:' header to specify the envelope sender, you can use 'sendmail -t' to trust the headers. Only do this for mail for which you really do trust the headers (i.e., don't use it in content filters where mail may have been BCC-ed or otherwise have headers that don't match the envelope).

As Evan said above, the mynetworks main.cf parameter usually controls which clients can use your server as a relay. See the BASIC_CONFIGURATION_README, SOHO_README or STANDARD_CONFIGURATION_README that comes with your distribution or via http://www.postfix.org/documentation.html.

Local submission via the sendmail command can use '-f' to specify the sender. Do check authorized_submit_users to make sure the nobody user can use local submission. The upstream default is

$ postconf authorized_submit_users
authorized_submit_users = static:anyone

but your distribution may set it to something else by default.

reset hostname, reset php and mail server. Actually you can reset everything related to web services, if that doesn't help then grep /etc for domain1 and wipe it, then reset again.