Many delivery failures are due to hyperactive spam filters at the ISP or desktop level that incorrectly intercept and trash email. The most likely candidate is a blacklist that may have incorrectly identified your sender address as a spam source. Blacklists are notoriously unforgiving, and it only takes a few spam reports, relayed to a central blacklister, for all valid emails from a given domain to be blocked. If the block happens at the ISP level, the misidentified email may be discarded with no notice to the end user, and with no way to recover the message.

Other filters focus on the content of your message, and the most common are based on a Bayesian formula. This is an algorithm that combines the probability of multiple, independent events into a single number. In the case of email, they figure out the probability of a message being spam based on the words appearing in it.

The filter scans through the message, looking for the presence of suspicious words pre-listed in its database. It then assigns a probability value to each word ranging from 0 to 1.0. A spamicity value greater than 0.5 means that a message containing the word is likely to be spam.

Spamicity is based on the number of times a word occurs in spam messages as opposed to the number of times it occurs in non-spam messages. For example, if a word has occurred 50 times in spam messages but only 2 times in non-spam messages, a message that contains it has a good chance of being spam. Common words, such as “for” and “what”, tend to occur about equally often in both spam and non-spam messages, so they have neutral spamicities.

The problem is, algorithms are imperfect and will mistakenly block some legitimate messages as well. Instead of sending out a long email, you might try sending briefer messages with a link to a Web page that contains the real message. With initial contacts, be un-spamlike as possible, and include information such as the domain or IP from which all your email will be sent to assist them in presetting their filters to let your mail in.