Due to the fact that there are so many advertisers and so few people who want to be advertised to, deliverability is an incredibly complex topic and there are several factors involved:
1.) Email Server Reputation - Web-based email clients check the reputation of the server sending the emails. If the servers have a bad reputation, emails sent from them will most likely be considered spam . You can check your email server's reputation here (http://mxtoolbox.com).
A sender's reputation is made up of a combination of IP Address + Domain + From Email Address so it's important that you maintain consistency. Some ISPs filter email by domain versus only IP address.
2.) Local Configuration Settings - Every email client and/or spam blocking application has its own local settings and configuration tools. In other words, users who receive email have the ability to customize their client to be strict or forgiving when it comes to spam. In our test, we have everything set to the default settings.
3.) Blocked Senders and Address Book - Several email clients use different levels of spam blocking based on if or if not the sender is considered "trusted" by the receiver. If the receiver places the sender into his or her address book, future emails are less likely to be blocked. When the receiver places the sender in their blocked list, future emails are blocked.
4.) Email Content and Subject Lines - Spam filters look for spammy sounding words in your subject line and the body of your email. Most work on a point system. The more points you have against you, the more likely you are to be placed into spam. Unfortunately most email clients will already start penalizing you for using HTML within your email.
5.) Mailing List Quality - If you have several subscribers in your mailing list that no longer have valid email addresses, you run the risk of getting blacklisted. This is why several email service providers are very strict about this. There are several affordable email list cleaning services available today and most email service providers remove individuals on your list who no longer have valid email addresses. With that said, it's best to stick with one provider and/or export your list of valid addresses if you plan to change providers.
6.) Engagement - Several ISPs keep track of the way their users interact with their email. Factors include what messages are opened, what messages are moved between folders and how quickly mail is deleted.
7.) Sending Server Configuration - Rate limiting will occur for high complaints and poor reputation. Servers should be configured to allow for simultaneous connections with throughput set at 20 emails per connection. Authenticating email with SPF and DKIM is recommended.
In the end, our system can only test the reputation of the sender address, subject lines and content using the default spam settings that come standard within email clients and popular spam filters.