Postfix mail server can stop spam coming from @mydomain.com (fake) to @mydomain.com. If you use such server, it is possible to stop this fake emails coming from your addresses (apparently).
In general, is not a very good idea to block emails based on one criteria only (RBL, missing RDNS etc.). On long term, you will lose legit messages. Thus, blocking messages without SPF/DKIM will make more harm than good.
Logical filter systems are a better choice. These solutions score each email message on all levels (connection, header, content, attachment etc.) using various techniques for each level.
I sincerely apologize that our chat person answered in this manner. At EZZI.net we have recently had a huge increase in the number of SPAMMERS and are working extra hard these days to ensure we keep our IPs clean.
If at all you still have an issue -- or see something out of the ordinary, please inform us - send me a PM and I will address it immediately.
I did not see this post of yours till now, when I was doing a monthly search against our name.