[Princess Sakura]

Hey, I just got a job at a small business as a website design/general network technician. They hired me mostly because of my ability to make nice websites, but also wanted me to maintain basic things on the server (Backup, adding client computers) I can do that, but this week some of the employees here are reporting emails being "Bounced back".
I'm completely out of my depth here.
The emails they recieve back look like:

Final-Recipient: rfc822;Timothy.Clulow@dotars.gov.au
Action: failed
Status: 5.0.0 (permanent failure)
Remote-MTA: dns; [152.91.3.169]
Diagnostic-Code: smtp; 5.1.0 - Unknown address error 550-'5.7.1 Rejected due to SPF policy for sender sales@mydomain.com' (delivery attempts: 0)
Reporting-MTA: dns; outbound.icp-qv1-irony-out4.iinet.net.au

Please be patient, I'm still VERY new to this stuff but help would be greatly appreciated. I'll try to give you as much information as I possibly can.

[StumpedTechy]

Heres an RFC faqs -

http://www.faqs.org/rfcs/rfc1893.html

The sender is not authorized to send to the destination.
This can be the result of per-host or per-recipient
filtering. This memo does not discuss the merits of any
such filtering, but provides a mechanism to report such.
This is useful only as a permanent error.

Its sounding as though sales@mydomain.com might not be liked by your outbound.icp-qv1-irony-out4.iinet.net.au and it is spitting it back. Have you verified that what your system is setup for on your domain has thias as a valid e-mail address you can send to/from?

[Princess Sakura]

Yeah, well that's the problem, both sales@mydomain and marketing@mydomain are being rejected, which could be a result of a spam filter on our ISP, but I doubt it's anything to do with the people we're sending emails to. Both girls had been sending emails to these people weeks before I had even started working with no problems.
Is it possible that the ISP has installed some kind of new spam filter that could effect these emails? I don't like feeling like it's something I've done that I can't fix.