Deliverability best practices

When you send email to your subscribers, the mailbox providers that receive your messages have one goal: only deliver mail to recipients’ inboxes that recipients want.

These companies, like Gmail and Hotmail, decide whether or not to place your messages in the inbox or the spam folder through complex machine learning algorithms that weigh signals of unwanted mail, like spam complaints and lack of engagement, against signals of wanted mail like recipient opens, clicks, read times, and much more.

Through identifying and tracking the sending behaviors and patterns of senders and brands over time, mailbox providers learn which senders generally send wanted messages versus which senders of email are sending spam. The way the world’s many different mailbox providers view your brand’s sending practices is generally referred to as sending reputation.

In order to optimize deliverability and get the best inbox placement rates, marketers need to establish themselves as legitimate senders by developing and maintaining a good reputation with mailbox providers. While Simon Mail’s Deliverability Services team is available to help guide you through this, follow these best practices to build your sending reputation and prevent deliverability problems:

Use a dedicated sending infrastructure (IP addresses and domains)

Using IP addresses and sending domains that are dedicated to only your sending and not shared with other senders allows you to control your own sending reputation and more easily manage your email deliverability. Senders who are sending more than 50,000 messages each month typically benefit from a fully dedicated sending infrastructure.
All Simon Mail senders use dedicated IP addresses and sending domains.

Authenticate your emails properly

Authenticating your email messages helps mailbox providers verify that the email messages you are sending are legitimate and not forged. The three types of email authentication are SPF, DKIM, and DMARC. These protocols help to prevent email spoofing and improve email deliverability through increasing both receiving server and recipient trust in your messages.

Simon Mail's Deliverability Services team assists all senders with authentication implementation and strategy.


Recipient spam complaints

Most spam complaints are generated because recipients don't recognize the email as coming from a sender they intentionally sign up to hear from. With mailbox providers considering any rate of spam complaints greater than 1 complaint for every 1,000 messages they receive from you as elevated and a sign of abuse, it's important to only send to recipients who have opted in directly and specifically to hear from your brand.

  • Never purchase, rent, borrow, or append email addresses.

Recipient engagement

Outside of recipient spam complaints, the largest negative signal that mailbox providers factor when evaluating sending reputation is how engaged, or not, recipients are with your messages. Continued sending to addresses that are not engaging positively with your messages tells mailbox providers that your mail is growing unwanted by recipients and will increase the amount of your mail that is filtered to the spam folder:

  • Stop sending to recipients who aren’t there or just don’t want your mail.
  • Remove addresses that have hard bounced in the past, registered spam complaints, or unsubscribed from your communications promptly and permanently. Simon’s Subscription Center automatically handles this for you.
  • Reduce the frequency of mail you send to addresses that are becoming unengaged so that you send to lesser engaged recipients less often than highly engaged subscribers.
  • Use purchase, conversion, or other first party data points along with email engagement data to help you identify your subscribers that are most likely to engage with your mail.

Send good content

When it comes to generating and maintaining a positive sending reputation, there is no substitute for content that recipients value and engage positively with. Some helpful points to keep in mind are:

  • Use a from name and from address that recipients are likely to recognize as the sender they signed up to hear from.
  • Do not hide the sender or use a misleading subject line.
  • Make your unsubscribe mechanisms easy to find for recipients and never try to hide the opt out link
  • Make subject lines clear and compelling
  • Use emojis in subject lines sparingly and judiciously
  • Keep your content as brief as possible with 1-3 calls to action
  • Include all relevant information for compliance with CAN-SPAM and other email regulations
  • Design content with mobile email experiences in mind
  • Ensure your content is accessible to all recipients through good html, appropriate color palettes and contrasts, use of alt text, and appropriate text sizes and spacing

Implement a Sunset Policy

Sending a lot of mail to recipients who never engage with those messages will increasingly drive messages to the spam folder. A sunset policy is logic that defines a point at which you'll stop sending marketing messages to an unengaged address. With Simon Mail, you have access to prebuilt sunset policy segmentation logic within the Simon Data segmentation tool to make crafting a sunset policy easy. (These segments are shared during your Simon Mail onboarding).

While a sunset policy doesn't take the place of targeting engaged recipients with engaging, personalized content, this logic does offer a layer of protection against sending to those addresses posing the greatest threat to your sending reputation.

Most mailbox providers consider a recipient to be very unengaged with a sender’s messages after 3-6 months of no email engagement.

  • Never send marketing messages to recipients who have not positively engaged with email in the last year.
  • Stop trying to deliver to messages that are repeatedly soft bouncing due to things like full or over quota inboxes.
  • Use Simon Data’s deliverability-specific data points to craft segment logic that only targets recipients who have been reasonably engaged in the recent past.

Monitor for deliverability problems

Good deliverability is simply when most messages you send are both delivered successfully and also reach recipients’ inboxes. The challenging part about making sense of email deliverability is that there are so many mailbox providers in the world that all function differently. Furthermore, those mailbox providers keep secret their methodologies for making inboxing decisions in order to protect their platforms from abuse.

Simon Mail’s Deliverability Insights make monitoring your sending results and identifying potential deliverability challenges easier by giving you quick access to meaningful metrics organized by critical breakdowns.

The most telling signals of potential delivery or inboxing challenges are:


  • Large discrepancies between delivery rates at different domains or mailbox providers often indicates that certain mailbox providers are blocking your messages due to poor sending reputation.
  • Poor delivery rates at all mailbox providers or domains often indicates a problem with how addresses are being collected or targeted as these are likely bad addresses.
  • Seeing significantly lower unique open rates at some mailbox providers or domains is a good indication that those mailbox providers are filtering messages to the spam folder more heavily than others.
  • Extremely low unique open rates that are consistent between most all mailbox providers or domains is a likely indication that recipients are not generally engaged with your messages and that general changes need to be made so that you are targeting only your most engaged recipients across all domains. Evaluate points of address collection and all content for optimization opportunities.
  • Unique open rates that are trending downward over time often indicate either decreasing engagement, increasing filter of messages to spam, or most likely both.
  • Distinct downward plunges in open rate can indicate an acute issue causing large amounts of mail to be filtered to spam.

Repair deliverability problems

When deliverability challenges are identified, it's important to take action to retrain mailbox providers to see your messages as wanted. Productive ways to repair deliverability problems once they arise are:

  • Isolate the mailbox provider or providers impacted by the problem and focus your repair efforts only on those providers. Use Simon Mail’s deliverability-specific data points to segment based on recipient mailbox provider or domain.
  • If your deliverability problem is a low delivered rate, examine your top bounce reasons for indications of block listings or references to spam. Look for any urls in the bounce reason that might provide instructions for resolution or more information.
  • Use Simon Data’s segmentation tool and your sunset policy to aggressively target only your most engaged recipients until you see open and click rates recover to the level you expect.
  • Engage with Simon Data’s Deliverability Services for advanced reputation repair strategies.