SMTP Server Restrictions

In order to provide the highest possible level of performance, our servers have the below restrictions when injecting messages.

  • Your IP address must have a PTR (Reverse DNS) entry.
  • Maximum of 10 simultaneous connections per account
  • Maximum 500 recipients per message
  • There is a limit of 50 messages per connection and sending multiple messages per connection is encouraged.
  • Messages larger than 128K are not DKIM signed.
  • Messages with an HTML body size greater than 128K are not open/click tracked. The
  • HTML body size is just the total byte count of of the HTML markup and does not include plain text bodies, attachments, images, or any other message content.
  • Messages sizes are limited to 30MB.


If you attempt more than 10 connections you will receive the following error: 
421 4.4.5 - Too many connections from your host