In a recent whitepaper, Spring Clean Your Email List: 5 Easy Steps,
marketing technology company Lyris touches on 12 ways to reel these
inactive subscribers back in.

1. Special offers.
   Consider offering discounts or free shipping for retail or a
   motivational whitepaper for B-to-B communications, suggests the
   whitepaper.
2. Survey subscribers. Find out why inactives have become disinterested in your e-mail communications.
3. Update profile. Using incentives, drive subscribers to their profile update pages where they can change preferences and personal data.
4. Understand their demographics.
   Many of your inactives could share common traits—they opted in as part
   of registering for the same whitepaper, seminar or promotion; a
   majority are the same sex, age, ethnicity, etc. Find ways to connect
   better with their demographics.
5. Try different send days/times.
   Different people respond better at different times of the day, week or
   year. Send out e-mails at different times to see if it spurs response.
6. Modify frequency.
   Considering adjusting your frequency. Maybe inactives are receiving so
   many solicitations that they’ve become annoyed. If so, tone it down. Or
   maybe interested customers are not getting enough e-mails to make an
   impression. In that case, try sending out more.
7. Create new content.
   Perhaps the content these inactives signed up for has become stale or
   their needs have changed. Offer different and new content to re-engage
   them.
8. Try different formats. Test using a text version,
   suggests the whitepaper, that is very simple but with specific links
   and messaging intended to drive action.
9. Test different subject lines.
   If the subject line doesn’t catch their attention, chances are the
   e-mail goes straight to the trash or junk folder. Try changing the
   subject line for inactives to see if it triggers response.
10. Monitor seed/proof lists.
    The whitepaper advises sending your messages to proof and seed lists
    for key domains. Monitor delivery in case content or images are causing
    your messages to be filtered or treated differently with specific ISPs
    and companies. If problems are detected, develop different versions of
    the messages that may not trip filters.
11. Send a postcard. Send a postcard that offers an incentive for inactives to update their e-mail preferences and profiles.
12. Move re-engaged to active status.
    After each e-mail message sent to inactives, change the demographic
    status of those recipients that clicked a link to “active.” This helps
    keep your focus on converting the inactives and tracking your success.

Source: Target Marketing