First of all we use Nolisting ("Poor Man's Greylisting"): no effort, much effect! After that we start our spam tests:
Because of the big amount of daily spam emails (UBE/UCE) we try to refuse as much spam emails as possible at SMTP layer so we don't have to deal with the content of the emails.
First all emails have to pass formal tests.
After the email has passed the formal tests we check the content of the email.
550 5.7.1 HELO something.local from RFC 2606 reserved domains blocked
because ".local", ".lan", … are no valid internet domains!
If you disable our spam filters (see below) you disable this feature, too, but it is preferred to change the settings of the Exchange server of the sender, which can be done within a few minutes!
If you use Outlook you should disable the junk filters, there and only use server side filters, instead.
Please read our error messages - we don't send them to the (assumed) sender but to the sending SMTP server!
If you want to report a problem: we need the IP address of the SMTP server, not the sender's email address!
Because it is more secure!