Email filtering and deliverability are based on many factors, including the following:

1. Complaints: Occur when subscribers mark your message as spam. Complaints happen for a variety of reasons, but may indicate that you have insufficient permission for some or all of the addresses on your list.  In addition, complaints can cause future campaigns to be blocked and sent straight to the spam folder.
2. High unknown user rates: When a message is sent to many email addresses that have become inactive or do not exist anymore.
3. Spam trap addresses: Email addresses that are used by a blacklist to test if the email address was obtained by a bot that is crawling for addresses.  If a message is sent to a spam trap, your IP reputation will drop.
4. Content: Spam filters look for specific words or characteristics from each message that are weighted based on their likelihood to indicate that a message is unwanted or legitimate mail. Spam filters are always adapting and learning more about what is and isn't unwanted mail, so it is not possible for us to offer specific advice about improving your mail content.
5. Sending Consistency: Sending with consistent volumes and frequencies month over month is ideal. Spammers tend to "pop up" on an IP and disappear. 

Here are some specific recommendations to consider that can help improve deliverability over time:

Complaints

• Clearly offer an unsubscribe option to subscribers and honor unsubscribes requests. Opting out should be just as simple as opting in
• Add text reminding subscribers where they opted-in to receive your email
• Monitor your complaint rates
• Validate you are adhering to applicable anti-spam and privacy laws and policies
• Ensure your marketing communications are timely, relevant, have been requested and that you have permission to send them to the user
• Consider the frequency of your mailings. What are the user’s expectations?
• Check if you imported new emails that were unverified or from an outside source. Email addresses such as info@, sales@, and webmaster@example.com are a clear indication that the list is not 100% opt in
• From address should be working email addresses that are well monitored

High unknown user rates

• Maintain your mailing lists. This includes purging old, bad or inactive addresses from your mailing lists. 
• Acquire names responsibly and send mail only to users that opt-in to receiving your email.

Spam trap addresses

• Monitor and manage both hard and soft bounces. Bounce notices can provide invaluable information regarding the ISP's treatment of your email.

Content

• Choose content wisely and verify URLs look normal and point to valid domains
• Use your own domain in your From address. Don't use free email addresses as your From address as all DMARC compliant mail receivers (including Yahoo, Hotmail, and Gmail) are bouncing emails sent as "@yahoo.com, @outlook.com, @gmail.com" addresses that aren't sent through their servers.
• Ensure emails are cleanly formatted and clearly identifiable as originating from your service.
• Use a friendly from name and email address to help subscribers recognize and trust emails sent from you
• Implement outbound email authentication using a sender authentication package. This helps protect from spoofing and ensure the Critical Impact mail servers are authorized to send mail, while protecting the brand and domain from threats to their brand and misrepresentation.
• Segment or separate traffic by brand or type of mail. Corporate email, customer acquisition, customer retention and transactional emails should be segmented. Senders who wish to maintain separate reputations for each should consider segment mail streams by IP address and utilizing separate authentication packages.
• Follow content and formatting best practices: use the Spam/link checker tool to help remove known spam keywords and broken links.

Sending Consistency

• Be consistent. Send email from the same IP's
• Less is more. Send less mail more often vs. lots of mail for short periods of time

The bottom line to remember is: if as little as 1% of your customers complain, the inability to communicate with your entire customer base may be the end result.

Finally, please thoroughly test before launching any campaign. This means frequent testing with recipient accounts using various clients and major email service providers to ensure that communications are being received in a desired fashion.

Deliverability Scenarios 

Scenario 1: Your email is being delivered to the Junk email Folder

Symptoms:

1.  Open, click and unsubscribe rates decline

2.  Recipients inform you that your email is delivered to the junk folder

Common Causes:

1.  Too many recipients reported your previous emails as spam.

Recommended Actions:

• Be consistent. Send email from the same IP's.
• Change the Subject lines of your messages to be more relevant.
• Make sure the From address is recognizable and would never be perceived as deceptive.
• Reduce the frequency of your messages.
• Make sure you are sending what the recipients expect to receive.
• Increase the relevance of your messages.
• Reduce frequency to addresses that have not opened or clicked recently.
• Make the Unsubscribe option easy to find and use.
• Send an immediate confirmation message.
• Add more relevant text content to your messages.
  
  

2.  Too much of your mail is sent to invalid or inactive email addresses.

Recommended Actions:

• Avoid mailing addresses that have not responded to your mail (i.e. opened or clicked) recently, or from users that have requested to be unsubscribed.
• To create an exclusion list, please follow these steps.

3.  Your sender authentication record is incorrect or missing.

Recommended Actions:

• Ensure your outbound mail is compliant. Contact support@criticalimpact.com to verify your sender authentication is set up correctly.

Scenario 2: Your message is delivered successfully without a bounce but not delivered to the Inbox or junk mail folder 

 Symptoms:

1. Open, click and unsubscribe rates decline

2. Recipients inform you that your email is not delivered to the Inbox

3. Some or all of the email sent to your personal email account is never delivered to the Inbox

Common Causes:

1. You are sending to dynamic spam/trap accounts

Recommended Actions:

• Reduce frequency to addresses that have not opened or clicked recently

• Segment mailings (transactions, newsletters) by IP address and try to identify bad data segments or sources

2. Too many of your email messages have been detected as SPAM by internal filtering and reputation systems

Recommended Actions:

• Change the Subject lines of your messages to be more relevant
• Reduce the frequency of your messages
• Make sure you are sending what the recipients expect to receive
• Increase the relevance of your messages
• Reduce frequency to addresses that have not opened or clicked recently
• Make the Unsubscribe option easy to find and use
• Add more relevant text content to your messages

Scenario 3: Your mail is blocked or mail is bouncing 

Symptoms:

1. You receive an unusually high number of bounce messages

2. Open, click, and unsubscribe rates decline significantly

3. Recipients inform you that your email is not delivered to the Inbox or the junk mail folder

4. Some or all of the email sent to your personal email account is never delivered to the Inbox

Common Causes:

1. Volume Caps

Recommended Actions:

• Utilize additional IP addresses with the same sender authentication

2. Blocklists

Recommended Actions:

• Contact Critical Impact support