550 This message was classified as SPAM and may not be delivered

This usually happens as the result of one of several reasons. Most notably, by not having SMTP authentication enabled in your email client. For example, if you are using the Microsoft Outlook email client, not having SMTP Authentication will cause several points to be added to the spam score. This can cause emails that otherwise would be allowed through to be classified as spam.

Another reason if you are getting caught by the filter is because of the message content in the title and body. If there are any words that are commonly associated as “spammy” terms, this will cause the message to fail if it scores high enough. This link will show you common and popular terms which are of a spammy nature and should be avoided

Why am I getting this error?
To help in the global battle of ever-increasing spam, we have implemented spam detection procedures on all emails prior to leaving our servers. Each email is evaluated and rated according to spam rules to determine how likely they are to be actual spam. Those that fail the test are returned to the original sender with the error message 550 This message was classified as SPAM and may not be delivered (please see the email below). If you are receiving a bounceback with this error, then the email you sent has not passed the outbound filter.

Common words that trigger spam filters
Below is a current list of the most popular words used in spam emails. As such, these words get higher spam points by evaluators. Once an email has too many spam points, it will get flagged as spam, either by outgoing filters or filters on the incoming mail servers. Using one of these phrases once will not trigger an email as spam, but using a phrase repeatedly or many of these phrases together in the same email can cause it to be flagged as spam.

List of most common terms that trigger spam filters