Share with your network!
One of the technical pitfalls that many senders face surrounds the rDNS of an IP address. rDNS (also referred to as reverse DNS or a PTR record) is the friendly hostname that an IP address maps to and, from a human point of view, can be used to more easily identify a connection.
Generally speaking, any IP involved in the transit of email should have proper and valid rDNS setup.
Why should you do this? Well, if you want to be considered a good sender, then you need to take as many steps as possible to separate yourself from the spammers. That starts with proper and valid rDNS since many spammers won't bother with this simple step. Furthermore, many ISPs do not accept messages from IPs with no rDNS set, and doing so may also impact your reputation with DNS blacklists and other anti-spam solutions.
What if you do have proper and valid rDNS setup and still have problems?
Increasingly, we are seeing a number of legitimate networks (web hosting companies, VPS providers, etc) whose IPs are being heavily abused by spammers. These IPs use the default and/or generic rDNS patterns that that network automatically assigns to them. Examples of this include but are not limited to:
hosted-by.example.com
server.example.com
static.example.com
where most every single IP in their network maps to the exact same hostname.
no-rdns-set.example.com
unused.example.com
unallocated.example.com
unknown.example.com
where the rDNS itself suggests that the IPs should not be being used for anything.
208-83-136-1.sfo.example.com
ip-208-83-136-1.example.com
where we see large volumes of spam from the network across multiple IPs and can see that the good senders have set their own rDNS up.
Another problem can stem from some senders having multiple rDNS records setup for a single IP. This isn't something we'd recommend, even if you have multiple users on an IP address. Such records are returned in a round-robin fashion and there is no way for you to guarantee which record a recipient network sees. As such, you may see strange and intermittent problems that are actually harder to diagnose from a deliverability point of view.
Cloudmark Sender Intelligence (CSI) uses a number of heuristics to determine the reputation of any given IP, in addition to the number of spam and legit reports that it gets. A lack of rDNS or particularly generic looking rDNS can be a contributing factor to that.
So if you have an IP that sends mail, make sure that you get some rDNS setup on it, and then make sure that you can set that rDNS to something that more closely identifies your organisation or brand.
For further information on rDNS naming patterns you may wish to consult http://www.spamhaus.org/faq/section/ISP%20Spam%20Issues#131