Email getting blocked by spam filters can be a real problem for you and your service provider. In many parts of the States, black lists are maintained and it is a tedious process to get you delisted and back in the good books of the recipients ISP. There are two forms of rejection: one that of an ISP or domain controller rejecting your mail and the other is where the sender ends up reporting your mail as spam. Most of the problems are caused by inappropriate subject lines and content in emails when it comes to mail being rejected by a service provider. There are also instances when you send too many emails to one domain in a day triggering off a spam alarm.
Usually, you will receive a message that categorically states that your mail has not been delivered because it is considered spam. These messages can also state that you have been blacklisted from sending mails again. However, in a large part, there isn't even a guarantee that this will happen because a blacklisted spammer will just start again using another IP; therefore, it makes sense to just subtly let the sender think that the spam mail is being delivered.
One of the surefire ways to know that your email is considered by filters as spam is to send an email from one of your accounts to another. You have to ensure, however, that these two accounts have never communicated before; otherwise, you can just set up a new webmail account. This is a method of testing that you can use. However, the same thing can be done by using any of the free-spam checker software on the net to check if your mails are susceptible to spam filters.
This is not really getting to the root of the problem and you need to understand how filtering mechanisms work and why some are more aggressive than others when it comes to filtering spam. Spam filtering is done with software that can have pretty advanced back end logic. Most of the advanced filters of today use a mathematical method to find patterns within each mail that is marked as spam. In the Bayesian method, keyword instances are used to calculate a score. Beyond a certain score, the mail is then marked for spam treatment.
This does not indicate perfection because many times spam filters will create false positives, which means marking the wrong mail as spam. This is also a cause of concern and must be factored in when you create your content. Never repeat a keyword more than twice and do not use similar phrases more than two times either. The last point to check is whether your subject lines are in sync with the rest of your content philosophy. This means that you cannot have a subject line which is filled with spam words, as this will simply end up wasting all the effort that you have taken in creating good mail content.
