If you have an email list you haven't sent to for months on end, should you treat it as a winback sequence? EG anybody that doesn't open or click after the reactivation campaign is suppressed? Or can you continue sending to them for 90 days and checking opens/unsubscribes? Ultimately, I want to avoid hurting my domain deliverability and don't want to send emails to those that don't want them.
The first step begins with the people who have not engaged with your emails, website, or any other content in a long time. Running such campaigns will not be easy for you have to make those inactive emails run through a reactivation series.
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First of all you really need to think about a hook to get people's attention. You cannot just try business as usual. After all you audience did not hear for you for some time and will therefore not be:
a.) looking out for your emails anymore and
b.) not knowing the sender or ignoring it
So, try a funny angle, a mea culpa angle, "a you would not believe angle", etc.. Anything that gets at least attention and whatever may fit to your audience. Also make sure to run an A/B test for the headlines. If you are using an automated e-marketing platform such as Active Campaign or similar, they have an in-built functionality for that.
Now to the second part of your question. There is no hard number for how long should you send emails until you delete someone from your list. But my consideration would be the following: You have once spent most likely a significant amount of time and money to create that list so I would not give up any member easily until it is clear he is definitely not interested anymore. Of course defining the last part is not easy. But I would consider several e-mail, over several weeks and then follow up with an e-mail explicitly asking do you still want to hear from me and an opt-out and reactivate link.
Hope that helps. Best of luck.
The first thing I would tell you is the main things which hurt your domain deliverability are too many unsubscribes or spam reports. If you're not getting those, then I wouldn't worry too much about your domain deliverability.
I would suggest coming up with something "free" to offer like an small eBook or guide. You can also run a contest and give something away for free to a subscriber. By offering something for free that has a high value, you can entice those people who may not respond otherwise.
From there I would continue to send your regular emails for 90 days to determine who opens/engages. After 90 days, I would send an email asking those who didn't open any of those emails if they want to stay on your list. Some email programs make this easy to track by allowing you to tag subscribers who click on a link. I use ConvertKit and they do this. Otherwise, you will have to ask them to reply back or maybe click on a special link. Wait at least 3 days before you delete those emails which haven't opened.
You will end up with a smaller list, but it should be a much more engaged list which will give you a much better ROI in the long run.