While some junk mail still gets through (“false negatives”). Filters are becoming a serious problem with many legitimate opt-in publishers being mistaken for spammers Email Database and blacklisted. Once on a black list, the only way to be unblocked is to contact the ISP and ask to be placed on a white list. While some e-newsletter publishing tools let you monitor how many messages are opened or bounced, you might never know which readers are being filtered unless you know which email program they're using.
Even then, the recipient might just be busy, uninterested, on leave or not receiving your message. Some filter programs don’t even notify you that your message Email Database was blocked while others bounce back mail with “sensitive" content without specifying the rejected words. On another occasion my own free weekly e-newsletter was blocked on its way to at least two Email Database subscribers thanks to Trend SMEX Content Filter detecting "sensitive content" yet the “naughtiest” word I could find was "teenage". Pity the poor medical publishers who often use “controversial” words such as “breast” or “penis” in their newsletters.
You might be able to track down the reason you were blocked by examining the full header information (ask your subscribers to forward the entire copy of your Email Database junked message). The header should list filter rules that the message violated, if the ISP uses software such as SpamAssassin. If not filtering, then what? Some online publishers advocate web Email Database services where you submit your newsletters for screening of content or technical problems that might trigger spam filters before mailing. Of course, spammers could also abuse such a service to get their own mail past the filters.