How Fast Can You Send Email?

For the best throughput, choose the most powerful server or cluster that you can afford. When speed (i.e., messages/hour) is a factor, a more powerful server will receive, process, and deliver deliver your messages faster. If budget is limited, a less powerful server may still get your message out in an acceptable time frame.

Dedicated Server Size

Micro Medium X-Large 2X-Large
Maximum msgs/hr 42,000 86,000 160,000 270,000
Spotlight Mailer Max/hr 32,000 64,000 85,000 100,000
Expected msgs/hr 20,000 40,000 80,000 160,000
Expected msgs/day 500,000 1,000,000 2,000,000 4,000,000
Recommended # connections 3-4 6-8 12-16 16-24

For increased capacity, high burst rates, or to account for other factors (see below) like large messages, lack of pipelining, etc., we recommend load-balanced multi-server clusters.

What do these terms mean?

Sending Rate

The sustained rate at which you can send email messages is based on many factors:

  1. Shared vs. dedicated
  2. Server power (CPU cores, memory and disk speed)
  3. Email sending program used
  4. Use of SMTP pipelining
  5. Use of SSL/TLS
  6. Proper use of concurrent connections
  7. Network speed between the sending program and LuxSci
  8. Message size
  9. Recipient server and DNS performance
  10. Blacklisting and greylisting
  11. Recipients/message
  12. Message encryption
  13. Other message processing
  14. Throttling
Maximum Rate The "maximum sending rate" (msgs/hr) is a theoretical maximum rate that you can achieve through the service. It assumes optimal sending conditions: very small (1 KB) messages, through a very fast network connection, all to the same recipient domain, where there is no blacklisting or greylisting, where SMTP Pipelining is used without SSL, where outbound encryption is not needed, and where the recommended number of concurrent SMTP connections is employed.
Spotlight Mailer Maximum Rate

The "maximum sending rate" (msgs/hr) for Spotlight Mailer is a theoretical maximum rate that messages can be sent from LuxSci's Spotlight Mailer under the same optimal sending conditions: very small (1 KB) messages, all to the same recipient domain, where there is no blacklisting or greylisting, where encryption is not needed, and where SSL is not used. Spotlight mailer is has an extremely fast connection to the SMTP server, uses SMTP Pipelining, and uses 4 concurrent connections.

Actual throughput with Spotlight Mailer will be less based on the other factors which affect the sending rate: SSL, Message size, Recipient server and DNS performance, blacklisting and greylisting, any additional processing being performed (e.g. anonymization or encryption), and others use of the Spotlight Mailer system (unless you have a dedicated Spotlight Mailer install)

Expected Rate Under realistic sending conditions, you may find your sending rate to be closer to the listed "Expected" rates, assuming that you also use SMTP Pipelining, 1 recipient per message, no SSL, have moderately small messages, no encryption, use a good email sending program, and use the recommended number of concurrent SMTP connections.
Expected Daily Rate This is an estimate of the number of messages that you could send in 24 hours of sustained sending at approximately the "Expected" hourly rate.

Sustained vs Burst Rate

For some customers, the speed of sending is very important. There are two significant modes of sending:

Sustained Rate

Sending at a consistent rate for a long period of time. E.g. 20,000 messages/hour for minutes or hours. Sustained rates are relevant when sending an email marketing blast to a large subscriber list.

Burst Rate

A "burst" refers to sending a bunch of messages in a very short time; e.g. 25 or 50 messages within a second or two. Burst differs from "sustained" in that when you are not "bursting" you are sending one or a few messages here and there ... infrequent sending. Bursting is relevant for transactional email messages when you are sending them "as needed" but require the ability to send many at once should that need arise, as it will.

Sending speed is important for customers who need to send large mailings at a sustained rate. Understanding sending speed helps you determine how long it will take to send that blast to 100,000 or 1 million subscribers.

Factors Affecting your Sustained Sending Rate

There is a theoretical maximum number of messages/hour that any server can manage. How fast you can actually send is affected by a number of factors.

1. Shared vs Dedicated

Dedicated servers will generally be faster. On shared servers, you must share the server's sending capacity with others and thus you do not have control over the capacity available to you at any particular time.

2. Server Power

More powerful servers can handle more concurrent connections and thus give you higher sending rates. The number of CPU cores is the primary factor governing performance; however, memory and disk speed are also important.

3. Email Sending Program

Different email marketing/sending programs have different capacities for how efficiently they can send high volumes of email. Simply switching from one program to another can increase your throughput significantly.

LuxSci Spotlight Mailer is a very efficient program.

4. SMTP Pipelining

Use an email marketing program (like LuxSci Spotlight Mailer) that supports SMTP Pipelining. With this feature, the program opens a single authenticated connection to the server and sends multiple messages in succession over that connection. This is significantly faster than opening a new connection and logging in again for every single message to be sent.

LuxSci Spotlight Mailer uses SMTP Pipelining.

5. Use of SSL/TLS

SSL is extremely useful for security and for the protection of your username, passwords, and message content. However, when using SSL, your email sending program and your server must perform work encrypting and decrypting all of your messages. This overhead will slightly slow down your rate of sending.

6. Concurrent Connections

If your sending program can open 2 connections to our SMTP servers at the same time and send two messages at once, then your sending rate can generally be up to twice as fast. Depending on how powerful your server is and how slow your network connections are, you can use more and more concurrent connections to achieve higher and higher sending rates. Combined with SMTP Pipelining, an appropriate number of concurrent connections goes a long way to maximizing your throughput potential.

The recommended number of concurrent connections for best results in sustained sending is given for each server type.

Note, that if you use more concurrent connections that is recommended, then you are likely over-working your server. That results in your sending rates actually getting slower. Stick to the recommended number of concurrent connections.

7. Network Speed

Your messages must be transmitted from your email sending program to our servers. The speed of your network, any packet loss, and the distance between your sending computer and our servers can significantly impact the maximum rate that you can send messages.

For example, LuxSci Spotlight Mailer sends from a server right next to your outbound email servers and thus network speed is not a factor. For someone connecting from outside of LuxSci or internationally, the speed can slow down significantly.

8. Message Size

Like network speed, the larger your messages are, the more time it will take to transmit them from your email sending program to our servers. For example, on a fast network, increasing message size from 1KB to 1MB decreased throughput by 50%. The actual impact of size will depend greatly on your network speed and other factors.

9. Recipient Email Server and DNS Speed

In order for our servers to deliver messages to your recipients, the servers have to (a) look up where to deliver the messages to (DNS), and (b) talk to those servers to deliver the messages to them. If the DNS lookup as slow or the recipients are in domains that no longer exist, this step can take a while. If the recipient's email servers are overloaded, on a slow network, or are having other issues, then the delivery phase can be slow.

Delivering many messages to recipients with these issues will slow down your entire sending process... as other messages have to wait for the slower ones to be sent.

10. Blacklisting and Greylisting

If your email content or sending patterns have gotten your server blacklisted or greylisted, then delivering email messages to recipient servers that are now blocking you or deferring you can slow down your delivery rates.

This can also cause a backlog of messages that are waiting to be accepted by these recipient servers. The need to Retry the backlog slows the process of delivering all other messages (though it does not block their delivery).

11. Recipients per message

Most email marketing and transactional email messages are sent individually to each recipient. This allows easier tracking of opens, views, etc. However, if you send each message to many recipients (e.g., 100s of recipients), then your sending program has significantly fewer messages to deliver to our servers. The delivery of your messages to our servers can happen 10s or 100s of times faster than if you sent in the usual one recipient/message pattern.

Additionally, if each message contains only recipients in a specific domain, then actual delivery to these recipients can be expedited by this efficiency. E.g. If you are sending to 500 AOL users, instead of sending 500 individual messages, send one message with all 500 AOL users listed as "BCC" recipients. Delivery can be almost 500 times faster.

12. Email Encryption

It takes some system resources (CPU cycles and network requests) to auto-encrypt outbound email messages. If all of your messages need SecureLine outbound encryption, this will somewhat detract from the maximum delivery rate. The impact will depend on many factors such as which encryption modes are used, if DNS additional lookup need to be made, etc.

13. Outbound Processing

If you are using SMTP Anonymization or other email processing features that come with Premium High Volume (e.g. content scanning, recipient restrictions, size restrictions,etc.), this processing will affect your maximum sending rate.

14. Throttling

LuxSci has features that can be used to rate limit the processing of outbound email. This is usually used when "warming up" IP addresses to assist senders in sending slowly and establishing their IP address reputation.

This rate limiting is on, your maximum sending rate will be capped by this process.

See also:

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A technical guide to bulk emailing

Book 3 in the LuxSci Internet Security Series.

Created by Erik Kangas, PhD

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