We were recently asked to comment on why a company's emails were not arriving at their destination. The company in question had recently sent out an invitation email to a list of clients and subsequently ended up on a black list. This was our advice.
It’s difficult to make a complete assessment since we don’t have some relevant technical details however there are several possible causes:
a) Local black lists: If you have been black listed once then it’s possible that email systems at the recipients sites could have cached this information and it will take some time before your IP address is removed from the local black list – it’s possible that it may never be removed in a small number of cases on poorly managed sites. I think this is least likely of the possible problems but I include it for completeness.
b) Junk Inbox: Email could be ending up in the personal junk mail of recipients. This is the downside of junk filters, they use weird and wonderful algorithms to identify junk mail characteristics. This often means that legitimate emails end up in junk mail. If an email goes to junk once (for whatever reason) then other different email from the same recipient may also end up in junk. There are a couple of ways around this: subtly changing email address policy (for instance tony.sprague@crmtechnologies.com instead of tsprague@crmtechnologies.com) then make sure that the recipient adds your address to their safe-senders list.
c) SPF records. This seems to be emerging as the standard means of identifying legitimate email senders (there are other methods SenderID and DomainKeys). SPF is the easiest to set up. Again I think this is unlikely to be the root of the problem, unless you have an incorrect SPF record in which case this would be bad. You need to talk to your technical guys about SPF and if its set up for the DNS record for your email server http://www.openspf.org/
d) Reverse DNS look ups. Make sure you have “reverse DNS” looks ups to your Email server. You need to talk to your ISP for this. They don’t know you need them unless you tell them. Some corporate email recipients may block emails that cannot be identified by reverse DNS. http://en.wikipedia.org/wiki/Reverse_DNS.
e) Open Relay. It’s possible that you were placed on a black list because you had a period where your email server was an open relay. This is like a hole in your firewall that is exploited by spammers. They use your email server to send thousands of emails. You get blacklisted or you close the hole then they move onto some other poor unsuspecting sod. In my experience this is the easiest way to get on a black list. To be honest, sending a load of invite emails from your outlook email address is an unlikely reason to be blacklisted. Black lists make use of thresholds which are sometimes in the hundreds of thousands of emails. http://en.wikipedia.org/wiki/Open_mail_relay
f) Domain Name : Backlists use IP addresses to identify the source of spam so its unlikely to be the domain name. Although the heuristic algorithms in some email content filters maybe prudish bordering on puritanical.
g) Delivery monitors. There are several services out there that can check your deliverability against blacklists and the most popular junk mail filters. Lyris.com has a good one. These are most useful if you are sending lots of emails.
Conclusions: Personal Junk folders is the most likely issue you have. Because you ended up in the junk folder some point (probably due to a corporate junk filter) the personal junk filter (most often on Outlook) may remember this and score you highly. This would also account for the fact that some people at an organization get your email whilst other do not – the ones who get your email probably didn’t receive email from you whilst you were black listed. We had a similar issue when testing an email blast with some high scoring spam characteristics in the content. The first email went into Junk. From then on all subsequent emails arrived in the junk box even thought we changed the content. Try subtly changing your email address to test this.