One or more of the following factors could be increasing the email’s spam score and causing it to be filtered into the spam folder:

• Suspicious content. Avoid spam-typical words like Rolex, Viagra, and casino in both the subject line and the email’s content. Even words that don’t seem immediately suspicious - such as “free” - may be marked as suspicious if they appear multiple times. An “unfortunate” combination of words may also be mistakenly marked as suspicious by a spam filter, leading to an increased spam score. Most often, only a thorough test by process of elimination can identify the part of the email that led to its classification as spam.
• Suspicious subject. Avoid ,e.g., question- and exclamation marks, especially in combination with CAPITAL LETTERS, €-icon, $-icon
• Phishing Links. Avoid spelling out URLs as plain text in HTML emails and linking them to a tracking link. This is common practice in phishing emails. Example: “Please register on www.promio.net”. Better alternative: “Please register here”.
• Too many emails per unit of time. Emails with identical or similar content were sent repeatedly in a short period of time (e.g., while sending test emails). Typical spam mails are often sent frequently within a short time. For this reason, some spam filters filter out emails of this kind.
• Unknown or suspicious email attachment. A file attachment is regarded as a potential virus carrier.
• Suspicious image-to-text ratio. The email is graphics-intensive. Whenever possible, convey your sales message in the form of text rather than images. Use more text, small images, and as few images as possible. Of course, this must be balanced against the sales goal!

Generally speaking, it is often difficult to reconstruct why a particular email was classified as spam. Frequently, the email is marked according to personal rules that the email client in question (e.g., Outlook) has learned over time.