You can use email marketing software to automate the email sending process. Email sequences can be set and let it run. For example, you can setup a series of email messages to be sent out automatically to subscribers. So, when a subscriber started the subscription to receive email from you, they will start receiving emails at a time interval set on the email software. Sequence can be set by time or it can be set based on whether the subscriber open the email or not. If it is set to be sent in time sequence, then the subscriber will receive email, starting from the time of subscription. This way, if you set the time interval for each email to be every 3 days, then the subscribers will receive your preset email every 3 days, whether they already opened your email or not. Alternatively, you can set to send your email only when the subscriber opens the 1st email, then it triggers the 2nd email to be sent in a few days later. This way, your subscriber will not get all your messages read in the order you set.