How to Speed Up Email Delivery

If you are experiencing slow sending speeds, there are a few things to consider.

The SMTP protocol can often require many "round-trip" cycles, where your computer/server has to wait for a response from us, and vice versa. Normally the best way to see improved speed is to do the following:

  • Open up multiple simultaneous SMTP connections (we allow up to 40).
  • Send multiple emails on each SMTP connection. Some software only sends 1 email per SMTP connection, then disconnects and reconnects, creating a lot of additional overhead. We allow up to 2,500 emails to be sent on each SMTP connection.
  • Use the SMTP Pipelining protocol. This speeds up delivery of emails that have been addressed to more than one recipient.

Example software that supports the above is Postfix.

If you are sending emails that are quite large (e.g. emails with large images or attachments), then the upload speed of your internet connection can limit your sending speed. Consider the following:

  • Reduce the size of your images and attachments. Images should generally be under 100KB in size.
  • If your attachment is larger than 1MB, consider providing a link in your email to the file rather than emailing the file as an attachment. You would upload the file to your webhost and include a link to the file in your email. A service such as Tresorit can let you upload large files. You can then place a link to the file in your email.

To get a rough estimate of your likely upload bandwidth capacity, you can perform a speed test at The important figure to pay attention to is the Upload Speed.


Keep in mind that if you are sending a 100KB attachment, your email will be larger than 100KB:

Attachments grow by about 40% when encoded using Base64.
Attachments grow by a variable amount (0-220%) when encoded with Quoted-Printable, depending on how much is ASCII and how much is not.

If you are seeing delays with sending to email addresses at your own domain name(s), you may need to whitelist our IP address ranges with your incoming (MX) server.

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