Email best practices

Tips to prevent users from unsubscribing or marking emails as spam

Use the email best practices below to encourage a more positive user experience and a healthier sender reputation.

User engagement is of the biggest factors for ISPs when determining your sender reputation, which is critical for successful email delivery and inboxing. Positive engagement, such as opens and stars, can help improve your reputation, while negative engagement, like marking as spam, can hurt your sender reputation.


Always comply with CAN-SPAM and CASL for marketing emails.

Audience selection

  • Before importing and targeting users in Leanplum, consider running them through an email validation service.
  • Target users who have:
    • Opted-in to receive promotional type emails.
    • Engaged or opened an email in the last 6 months (or a length of time you determine). Users who are less engaged are more likely to engage negatively.
  • Set user expectations during sign up by letting them know:
    • The type of promotions they’ll be receiving.
    • The frequency in which they will receive the emails.
  • Avoid targeting users with addresses from affiliate vendors or were purchased. These users did not explicitly opt-in.
  • Increase sending volume by ramping up gradually, using our warmup schedule when introducing a new segment.
  • Target users who have not engaged in more than 6 months, once a quarter, or twice a year. Then permanently sunset this population. They may be negatively affecting your sending reputation and hindering your engaged population from receiving your campaigns.


  • Be sure to use your Leanplum requested sending domain.

  • Target users with subject lines, preheaders, and content that are relevant to their segment.

  • Include an unsubscribe link in all emails with marketing categories.

  • Ensure content encourages positive vs. negative recipient engagement.

    Positive Engagement
    Moving from the Spam folder to the Inbox
    Reply to
    Getting added to the address book

    Negative Engagement

    • Marking as spam
    • Unsubscribing
    • Not opening
    • Auto filtering to the spam folder
    • Moving mail straight to trash
    • Mark as not important


There are a few things to keep in mind when using images in emails.

Dimensions and image quality

  • Most of the email clients have a preview pane of 600 - 800 pixels maximum width.
  • To support high-resolution (retina) displays, you need to resize images 2x the desired display size.

Image file size

  • Have in mind that more than 50% of emails are opened on mobile devices. Although it is recommended to use 2x (retina) images, increased image size might affect email load time and subscriber's mobile data plan, resulting in less-than-friendly user experience.
  • The optimal total image data weight should be less than 1mb.

Image file format
The three most widely accepted file formats are JPG, PNG, and GIF.

  • JPGs are the most compressed in terms of file size and therefore are the most suitable for photographs.
  • PNGs have a higher quality and can have a transparent background. However, greater quality leads to increased file size. That is why PNGs are recommended for smaller images that require greater color capacity, such as logos.
  • GIFs allow animated loops but are the most limited in terms of colors. These color limits, however, help file size to remain small. GIFs are mostly used for animated memes to convey a message in creative ways.


SVG Support in Email

SVG is also a popular image file format. However, it is not supported by Gmail and other popular email clients. Do not use SVGs in emails unless you are completely sure that your subscribers' email clients support this file format.

Alt Text
Alt Text is the text that appears in the case your image doesn’t load. It also improves the accessibility of your email. Make sure to use Alt Text with images to avoid empty image placeholders. In general, it is recommended to create emails with the assumption that images are blocked by email clients, or that some images might completely fail to load.


  • Analyze engagement metrics12 to 24 hours after each mailing and adjust targeting, content, and mailings frequency for future sends if needed.

Sending volume

Inbox Providers monitor your messages across a 30-day rolling period looking for anomalies such as sudden volume spikes, as well as long breaks without any mailings. If you want to keep a certain sending volume we recommend not going beyond 30 days without sending.