Spam is any email that is not wanted or requested by the recipient. While the majority of spam comes from an unknown sender, oftentimes legitimate email is categorized by spam either by the recipient or by the ISP delivering emails to customer inboxes. So why does this happen?
Recipients who label your email as spam do so by clicking the “This is Spam” button in their email client. This notifies the ISP that this is unwanted mail. Each ISP has different thresholds or limits for the amount of acceptable complaints a sender can have for each email deployment. If you go above that threshold, you are likely to get blocked in the future. Having a high complaint rate is the number one reason why your email delivery will get blocked or delivered to the spam folder.
Content can also be a factor. Your message may also end up in the spam (or junk) folder because they have characteristics included in the email that make it look like spam or is suspicious. For instance, emails that contain only images or contain email attachments may also be flagged as spam. This is because they may contain viruses. Be careful of using “spammy” words or phrases and make sure that your HTML email is coded properly. Broken images and missed tags are tell tale signs of spammers.
Finally, be transparent. Send your email from a valid domain name and authenticate your mail using SPF or DomainKeys. This helps ISPs properly identify legitimate senders.
ISPs have a tough job trying to keep spam out of the inbox, that they need a little help to identify the good senders. As long as you follow a few best practices, your emails will get delivered.