Deliverability requirements

Deliverability may cause some troubles when you don’t know exactly how it works. Find out what options you have to increase your CTR, learn how to avoid ending up in a spam folder and know what a spam trap is.


  1. Deliverability in a nutshell
  2. Mailbox providers give you different filtering options
  3. What is Spamhaus
  4. Spam traps
  5. What is feedback loop

1. Deliverability in a nutshell

Deliverability is about sending mail that users both want and expect to get. Obtaining consent from recipients, meeting their expectations, as well as creating and maintaining an excellent reputation for your IP address and domain are crucial for the process.

The mentioned process of delivering your email campaign should consist of: 

  • Authentication
  • Sending the mail your subscribers expect
  • Address capture and contact list hygiene
  • Complaint management
  • Frequency and engagement

There are four parties involved in the email deliverability process – the senders, the ESPs, the ISPs, and the recipient.

  • Sender is the company or the individual that schedules the email message.
  • ESP – Email Service Provider is a company that offers bulk email sending services. Besides the infrastructure which is used for physical campaign send-out, they offer also functionalities that help design, personalize and analyze the email campaign.
  • ISP – Inbox Service Provider is a company that offers the email inbox services. The most popular free ISPs are Gmail, Outlook, iCloud Mail and Yahoo! Mail.
  • Recipient – the person who has opted-in to receive the email. Usually, the individuals have to sign up to receive messages from various companies. 

That means that before your email can be delivered, it must get pass the ISP filters and requirements. 

2. Mailbox providers give you different filtering options

In terms of delivering the email, different mail programs have different filters and requirements for the messages they receive. Mailbox providers install filters in their mail transfer agents, that forward mail as a service to all their customers. 

Emails in different programs are sorted by filters according to specific criteria. Originally filters were mainly used to identify spam and block or move it to a particular folder. Nowadays, some mailbox providers use email filters to categorize messages in order to organize the inbox (e.g. Gmail categories or Microsoft’s Focused Inbox).

Mailbox providers use spam filters, whether they build their own system, use third-party spam filter technology, or use a combination of both.

As messages travel from the sender to the subscriber’s inbox, different types of filters can affect deliverability and placement in the inbox:

  • Gateway spam filters serve as the mailbox provider’s first line of defense in preventing spam. All mail attempting to enter the company must pass through this “gateway” before it enters the system.
  • Third-party spam filters are companies that have developed an own method to distinguish spam from legitimate mail. This kind of spam filters may affect filtering decisions at the gateway or after its acceptance by the gateway (i.e., placed in the inbox or spam).
  • Desktop spam filters are basically third-party spam filters but on your own computer.

Mailbox providers consider four main aspects of mail when filtering:

  • Mail source
  • Reputation of the sender
  • The content of the mail being sent
  • The engagement of the subscriber.

3. What is Spamhaus

To prevent spammers from sending as many emails as they would for sure want to, the need for blacklisting tools arose. 

Spamhaus is one of the public blacklisting tools, which except for just blacklisting, runs the Register of Known Spam Operations – the list of users marked as spammers three or more times. 

Its policy is based on the belief that the only way for email to remain a valid and useful channel is for users to receive only those messages that they themselves have requested. No email should be sent unless there is direct and verifiable consent to do so. Spamhaus decides what IPs or domains it lists or removes, which is helpful when it comes to spam traps.

4. Spam Traps

By sending your bulk email campaigns you can easily get caught in spam traps. 

A spam trap is simply put an old, recycled email created by ISPs or public blacklist providers which isn’t actively used by a real person. It is used in order to detect email senders that try to send emails to not verified, inactive users. Each sender has to make sure that the spam traps are not present in their mailing list, in another case it can heavily decrease their reputation.

Types of spam traps:

  • Pristine spam traps are email addresses that were never valid and may not have even opted-in to receive email.
  • Recycled spam traps were once valid email addresses, but have since been reused by their provider.

You should make sure to avoid spam traps by regularly cleaning your contact list. 

5. ISP Feedback loops

In case your mailbox provider won’t capture spam messages and it will land in your regular inbox, ISP (Internet Service Provider) Feedback Loops (FBLs) will allow you, as the ISP user, to report the message and its sender. By simply clicking the button “report spam” or “this is spam” you will inform your ISP that you find the given email unwanted.

Wrapping up:

If you want to maintain the best deliverability possible, not falling into the abyss marked as a spam provider you should just stay authentic, including your details in messages, send them keeping a healthy frequency, and only if your recipients are expecting your mailing. Keep your contact list clean and hygienic to take your email marketing to a whole new level and remember that your email content, engagement, and list-building methods can and will affect your deliverability. 

If you need more information about the topic mentioned above, please contact us: +1 800 960 0640