Allowing different "from address" to be relayed

[fxnick]

I have a contact form that users fill out on my website and it gets emailed to me.

This the error i am experiencing when the form is filled out:
relay=mail.XXXXXX.xxx[ip address]:587, delay=0.07, delays=0.01/0/0.05/0, dsn=5.7.1, status=bounced (host mail.XXXXXXX.xxx[ip address] said: 553 5.7.1 977@gmail.com: Sender address rejected: not owned by user nick@xxxxxx.xxx (in reply to RCPT TO command))

When the form is filled out the users email is used as the “from” address, and i think this is why MIAB is rejecting emails. When I use my email (nick@xxxxx.com) as the user email it goes through.
What needs to be changed to allow emails like this to be relayed?

This is last thing holding me back from switching over to my new server.

[murgero]

You cannot do this with MIAB.

From address must either A) Equal username or B) be an alias of said username.

I’d have the email in the Subject or body of the message and send using the email you setup for the contact form to authenticate with.

[fxnick]

So after looking at other posts here, I commented out this line from main.cf:

#smtpd_sender_restrictions=reject_non_fqdn_sender,reject_unknown_sender_domain,reject_authenticated_sender_login_mismatch,reject_rhsbl_sender dbl.spamhaus.org

And my problem has been fixed.
Is there any security risks for doing this?

[murgero]

Yes. Users can now pretend to be someone else. The whole reason that is in there is to stop someone like me as an example from sending email AS YOU.

Please use my solution above instead.