In certain cases, Inbox Service Provider’s(ISPs) limit the amount of email they accept from a particular sender during a specific period of time. If you send email above their threshold resulting in a high volume, your email will be rejected ultimately leading to a high number of bounces backs. This is called “Email Throttling” also known as “Email Deferral”.
 

Or we can say Deferred mail is an undelivered email added in the long email queue.
 

If an email is deferred, you will be able to see the following message in the user’s contact activity feed:

 

Although there is no such specification about the limits each of the ISPs applies, few factors can be considered which involves in Email Throttling such as abuse complaints, bounce rates, spam trap hits and subscriber engagements
 

a. High Volume Mails

It is very simple to understand that our body needs a proper warm up to build more strength and prevent injuries to our muscles. Similarly, a proper warm-up is necessary to prepare the sending domain and IPs for sending high volume emails and to prevent injuries or we can say blacklisting or blocking.

Specifically, for new IP addresses, it is difficult for any ISPs to authenticate that IP address and allow the delivery of emails. It is recommended to send emails in a consistent manner for some period of time from the New IP addresses. This is a process which can be termed as WarmUp Process. 
 

This warm-up process helps ISPs to know the sender and the sending behavior. Your authenticity will decide your score and your score will decide your reputation in ISPs. 
 

b. Bad IP Reputation

IP Reputation is the key point to be taken into consideration while sending emails. The better your IP Reputation, the higher your email deliverability rate. There are multiple tools to learn about your reputation. Sender Score in one such tool which scores your IP from 0 to 100 and shares the improvisions required by the sender. 

A bad reputation can block your emails. This is why a warm-up is very important for every single IP in your loop. Without a proper warm-up IP remains unidentified and ISP do not allow email practice from such IP. If emails are sent from such unidentified IP it will be blocked or land in the spam folder. Either way, the sender is in the loss. 
 

c. High Spam Ratio

Sometimes a few subscribers mark your email as Spam. Up to a limited number, this will not cause problems. Although even one 'Report Spam' is like an alert signal for ISPs. ISPs will note the behavior of all your remaining subscribers and if found suspicious, the email reception can be blocked.

Regular testing and analyzing will help you learn and improve to keep your reputation up. If you have 1 spam complaint, take proper and quick action to sort this as quickly as possible. ISPs like genuine and disciplined senders. 
 

d. High Bounce Ratio

Bounce ratio is another important factor to add positive and negative marks to the reputation of any sender. This is the responsibility of the sender of managing the bounce status on the database on which campaign will be sent. There are multiple reasons of high bounces and as per ISPs bounces above 5% can lead you to blockage.   
 

 

Preparation Against Email Throttling

a. Separate Your Marketing & Transaction Email 
Avoid using the same domain and IP network to send a variety of emails.

For example, if you are using Domain-M & IP-M for sending marketing emails then there is a possibility of spam complaints based on nature of email. Transactional emails, on the other hand, are important and one to one communication emails. Transactional emails cannot be compromised and if Domain-M & IP-M are used then emails delivery will be hampered. 

Make sure you use a separate set of domains and IPs for each category of emails. 

b. Proper Warm Up

It is very important to ensure that each new IP and domain is properly warmed-up before sending emails in high volume. This accolades the domain and IP and credence the ISPs. 

Keep your sending limit lower initially and gradually increase the limit. This will help ISPs identify you as a legitimate sender thus approving your email reception. Warm up is a slow process but will build a reputed history for future email practices and better performance. 
For more refer: Warm Up Process
 

c. Clean-up Your Database

You made sure to manage your IPs and domains but if your database is not clean then it will all be destroyed with just a few emails. Bad data will present an impression of unsolicited mailing by the sender.

So, make sure you clean your data of inactive, non-existing email ids, unsubscribes and also remove those who are not responding for a long time. Use only the opt-in set of data which is not 6 months old. Keep pampering your subscribers with regular newsletters. This will help you keep your database fresh and up-to-date. 
For more refer: Keep Your Data Clean. Find Out How
 

d. Avoid Bounces

While sending emails in high volume it is recommended to manage both hard bounce and soft bounce ratios. It is always suggested to send your emails to active and recent opens & clicks. The first time your send mail on a list there will be opens, click-through, forward which decide the total interaction of receivers over that mail. Higher the interaction, higher the reputation. 

Now, you get a certain ratio of bounces as well. Remove them or suppress them so your next mail does not include these email ids. This will lower your bounce ratio. 

1. Never send emails too often on “soft bounce email addresses”.
2. Ensure hard bounce rate of less than 5%.

Email throttling is a normal process and it is quite difficult to find out the rules of ISPs. Above this, each ISP follow a different set of rules. All you need to do is practice legitimate mailing and once you are identified by ISPs as a reputed sender, you may not face the throttling or deferral or blockage issue.