Segment your contacts
While there are appropriate times and emails to simply send to every contact in your contact list, because it yields higher response, segmenting your contacts into meaningful groups and sending targeted emails to the groups based on the nature of the group will be much more effective and prevent abnormally high unsubscribing.
Segmenting is all about knowing your customers, researching trends statistically, and effectively using reports built into your store’s admin. For instance, you want to send a campaign to customers who have not purchased in the last six months. (instructions for how to do so and create a segment in the tool)
Be consistent without being obnoxious
Send emails in a frequency that doesn’t surprise your contacts
Know your audience
There’s a reason most businesses that use email marketing successfully don’t outsource their email marketing. No one knows your customers like you know your customers.
Be purpose-driven
While simply sending some sale product or service on a monthly basis is much better than never sending any email marketing, the email marketers who consistently have great click-through rates and response to their campaigns are those who plan ahead and are purposeful about their campaigns and the audience for those campaigns.