In my current strategy using Postfix, I employ the following techniques:
a. Separate Rules for Each Domain: I create distinct rules for domains like Gmail, Hotmail, Yahoo, and others, not only for sending but also for logging purposes.
b. Simultaneous Sending to Unique Destinations: The strategy involves simultaneously sending emails to each unique destination with varying rates. These rates are adjusted based on logs and the outcomes of trial and error.
c. Separate From: Subdomain for Each Category: To enhance deliverability, I employ distinct From: subdomains for different categories of emails.
d. Destination-wise Rules: I would advise to create destination-wise rules. For details refer to my article The Concept of Destination in the World of Email Deliverability
e. Tailored Rates at Multiple Levels: I customize the sending rate for each individual domain and mailbox within each domain.
Tailoring the Rate of Sending – A Complex Endeavor:
Determining the appropriate rate of sending emails is a challenging aspect of mass mailing. Several factors come into play:
a. Sending Rate: The speed at which emails are sent is carefully managed to avoid triggering alarms on the receiving end.
b. Domain and IP: The domain and IP used for sending play a crucial role. Simply changing IPs while using the same domain to bypass restrictions is not effective. Services like Gmail may block the domain if they detect excessive email bombardment from multiple IPs associated with the same domain.
c. Gmail’s Content Filter: Gmail has implemented a stringent content filter that swiftly blocks emails based on their content.
d. The Multiple Sub-Domain Strategy: Utilizing multiple sub-domains can be an effective strategy, which we will discuss in more detail later.