In short: Mailing list pruning means removing invalid and idle contacts so your list stays full of people who want your emails. Find your inactive, invalid, disposable, and role/group addresses, use the Zoho Campaigns contact views to spot inactive contacts, and try to win idle contacts back before you remove them. A clean list improves engagement, domain reputation, and ROI.
The success of an email campaign vastly depends on having interested and engaging contacts in the mailing list. Marketers spend time and money to keep their mailing lists hygienic as old and dirty mailing list can directly affect email engagement and deliverability.
Mailing list pruning – An infographic.
What's a hygienic list?
A hygienic mailing list is the one with regularly engaged and interested contacts. By regularly pruning invalid and idle contacts, you are halfway through your cleaning process.
Importance of a hygienic list
Allows you to closely focus on contacts who really have interest in your product and create content that satisfies their needs.
Avoids bounce rate, spam complaints, unsubscribes, and spam traps.
Gives you higher conversion rates.
Improves your domain reputation as it's a positive signal to recipient systems that you're committed to respecting your recipients inboxes rather than spamming them.
What if you keep your mailing list messy?
A dirty mailing list may seem huge in size, but it includes invalid addresses, typos, and idle and uninterested contacts. When you send emails to these addresses, your emails will bounce and receive spam markings and unsubscribes. This will severely affect inbox placement for contacts who regularly read your emails and reduce your ROI. Google asks senders to keep their spam rate below 0.10% and never reach 0.30%, and Yahoo recommends staying below 0.3% as well. See how to avoid complaints and bounces. How to prune a list
There are four kinds of people you should focus on in your mailing lists:
Unresponsive since signup
A few contacts might have never engaged with your emails right from the time they signed up. These email addresses are mostly invalid or typos. Sometimes leads would have signed up out of curiosity and would have never checked your emails. They offer limited chances for winning back, and it's recommended you use a list cleaning service like Bounceless, Zerobounce, or Pabbly to remove these addresses from your list.
Once active and interested but now idle
Contacts/leads who were once active often become inactive because you didn't continue to send them the sort of content they expected when they subscribed. Also, when you send emails that are verbose or contain a lot of images, the content might look spammy. When leads/contacts keep receiving such emails over a period of time, they stop reading them and become idle.
Secondary/disposable addresses
People use temporary or disposable free email addresses and subscribe to your mailing lists when they want a free trial of your product/service. These addresses may expire soon and become spam traps. You can add the update profile link in your emails and ask your contacts to update their email address. This will help ensure you're sending emails to your contact's primary email account. See spam trap effects. Role/group addresses
Role (hr@ and manager@), group addresses (admin@ and support@) and Postmaster (postmaster@example.com) email addresses are deliverable addresses, but they are problematic in the following ways: The address may be shared by many people in the team/group who may not know you and may mark your emails spam.
When you send emails to Postmaster email addresses, your sender domain might be blacklisted.
We recommend you avoid these addresses and advise you to get your contacts' primary email address.
How Zoho Campaigns helps you
Zoho Campaigns provides predefined contact views that show you inactive contacts. To view them: From the Navigation Toolbar on the left, click Audience and select Contacts.
At the top-left, you'll see the contact views, such as All Active Contacts. Click the More Actions button to see more views.
Click one of the following views to see inactive contacts:
No emails opened last month
No emails opened in last 3 months
No emails opened in last 6 months

Inactive contacts refer to those contacts who haven't opened or viewed your email campaigns for a particular time period. For example, the "No emails opened last month" view lists contacts who haven't opened your email campaigns in the past month. These views are based on opens.
For longer periods or other criteria, you can click the Filter button at the top-right of the Contacts page and use the Email Engagements category, or create a custom view: click the More Actions button next to the contact views, click Create Custom View, enter a View Name, choose your criteria, and click Create View.
Also review the Invalid Contacts, Bounced, and Unsubscribed views. They list contacts with unusable addresses, hard bounces, and opt-outs.
You can also use the Contact Scoring and Segment features to identify inactive contacts. First, you have to assign scores for your contact based on their activity. Next, you can create a score-based-segment to identify inactive contacts. Contact scoring is available on the Professional plan and on the Zoho bundle plans that include Zoho Campaigns. Once you have identified inactive contacts/leads, you can use the following techniques to convert dormant contacts to an active state:
Send re-engagement emails
The best way to win back idle contacts is by sending a re-engagement email. In this email, you can spotlight the benefits of being on your mailing list and offer exclusive discounts as a goodwill gesture. For those who didn't respond to your re-engagement email, you can send a confirmation email for opting out and provide an option to resubscribe in the footer. Make the unsubscribe option easy to find: Gmail and Yahoo require senders of 5,000 or more emails a day to support one-click unsubscribe, as defined in RFC 8058, on promotional emails.
Offer the right content
Contacts become inactive if they don't find value in your emails. You can include a topics link in your email footer and give your contacts the option to choose the topics they want to hear about. You can then send personalized emails about the topics your contacts have preferred to receive. This will improve email engagement, and your contacts will want to hear more from you. Optimize your email frequency
Email deliverability and domain reputation will take a major hit when contacts don't read your emails. As there is no point in sending frequent emails to inactive contacts, you can optimize email frequency to prevent further damage to domain reputation and email deliverability. Initially you can send two emails per week instead of sending emails on alternate days. If your contacts still don't open your emails, you can send two to three emails per month. If your contacts have not opened your emails for more than three months, you can send an email to understand if they are interested in receiving further emails and provide the option to unsubscribe. You can cap how often each contact is emailed with frequency capping. Sending targeted emails
Sending emails targeting your contact's interest is one way to keep them engaged. You can identify your contact's interest by analyzing the searches they perform on your website, links they click in your emails, enquiries they make about your product/service, and their purchase history. Sending targeted emails will not only help you win back idle contacts but also improve email deliverability and domain reputation.
Cleaning your mailing list will work wonders for your email campaign as hygienic mailing lists contribute to 40% of your email campaign's success and boost email engagement, domain reputation, and ROI.