I-World - Internet Services and Information Management
This seems to be a reasonably accurate validator, at least for individual email addresses. We have not used it for a bulk list. We did use another service and it claimed good addresses were bad, which means we removed addresses of supporters that wanted mail from a non-profit. Since that incident we do not trust validators in general, but if this one is used for single addresses the output is reasonably verbose. It can be helpful as a complement to other information.
This site offers all kinds of analyses and checks, including a blacklist check on about 100 lists. (However, we inherited an IP with a sketchy history and found that even though it was clean on these blacklists and this check it was blocked by in-house lists at AOL, att, and a few others. This does not show up here.)
Also be aware that a few things that show up as errors here are not really errors, in one case an "error" that was beyond our control turned out to just be a failure to follow a common protocol which is not mandatory. This "error" has no consequences. So problems on mxtools still need to be investigated and may or may not matter.
This is an extensive list of IP numbers that are never up to any good. They are not end-user IP's but blocks assigned for servers and things like that. We use it to block web access but it is probably useful for blocking everything. We may begin to use it at the "front door" in the firewall.
We recently added SPF and DKIM on two servers. This article on using opendkim to sign outgoing mail and verify incoming mail is extremely good. It can essentially be followed precisely with no problems, on FreeBSD. We had two or three minor issues, not the fault of these instructions. First, make sure that if you copy and paste the key generation shell script all the quotes are in place. Second, make sure the dkim TXT entry in the dns record is right. This is not covered well here, look it up. The domain/host entry needs to have the right form as a subdomain. There are verifiers to check this.