Delays (Temporary Failures or 4xx Errors)
When we receive a 4xx rejection from the Internet Service Provider (ISP), we log a "delay" event that includes the timestamp and the attempt count (example: "num_retries":"6") and retry again later. The proprietary retry algorithm attempts delivery for each message a certain number of times over a time period; the process is dynamic but it would not be unusual for a message to be retried up to 6 times over 72 hours.
If we hit the maximum number of retries (or maximum time period the message is eligible for delivery) and the remote server still has not accepted the message, the message will be failed with a timeout reason – in this case we will log a "bounce" event. The bounce classification routine will use the last 4xx response string to determine what type of bounce should be recorded.
Note: These types of bounces (those for which the remote server continually rejects the message with a 4xx error) are not usually classified as hard bounces they are soft bounces.
There is also another type of delay that we may see:
"reason":"451 4.3.0 [internal] Sending IP temporarily suspended"
These delays are due to the Adaptive Email Delivery (AEN) doing its job. The AEN is an integral part of email delivery that moderates traffic to comply with ISP/server rules and feedback. When we get push back from them, software detects these bounces and takes the appropriate action to "back off" of the receiving endpoint. Listening to feedback from ISPs allow us for a better reputation, resulting in higher deliverability for everyone on the platform.
ISPs delay messages for multiple reasons such as mail server busy or over loaded at the time of send, etc. Like other Delays, they are retried until delivered or timing out after 72 hours.
The AEN is only acting on the behavior of the ISP and is meant to hold onto your message until the ISP is able to accept it rather than receiving a block bounce and not delivering at all. It works on the concept of better late than never.
It is generally not a good idea to send huge amounts of emails if one go if you have not sent a lot of emails previously as a ISP delivering your emails may view these as spamming. You would effectively need to "warm up" the ISP (hotmail, gmail, talktlak etc) to you sending emails to them as sending a whole load all of a sudden with no previous sends could lead to lots of your emails being classed as spam or being delayed.