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Email Throttling


Email Throttling is a process in which you limit how many emails you send to a specific ISP over time. If your sending a large volume of email to an ISP really fast (bulk mail), it will set off "Spam Filters" and they will start delaying, bouncing, or rejecting your emails until a period of time has passed, before it will accept email from you again. This happens often with new IP's that haven't been Warmed Up.

There are many programs that allow adjustable throttle rates, that will batch up your emails to a particular ISP and only send X number of emails per minute. This allows you to maximize your delivery rate while Warming up your IPs. If you're just sending normal workflow email, you shouldn't have a problems; it's when you bulk mail a "Newsletter" or an "Update" to everyone at once.

How do I know what email throttle rates to set?


Every ISP is different and no one publishes that information; some won't accept more than X emails per hour, some won't accept more than X emails per minute. Every ISP is different, but what is known, the slower you send the better your success rate. Once your IP’s are warmed up, these initial rates adjust to allow greater mail flow.

Open Sourced email throttle rates.


There is an open source project for a plug in to PHPList (a free newsletter mailing program), that people have contributed to would allow you to better group related domains in order to Throttle more effiencently. We at Unlock The Inbox have made several contributions to this plug-in (https://github.com/michield/phplist-plugin-domainthrottlemap).

Is there another way to increase my send rate initially?


If you get your IP whitelisted at an ISP or sign up with an "Email Certification Service," then you might be granted the luxury to send without throttling or warming up your IP's Addresses to certain ISP that have a relationship established with the Email Certification Service.

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