Setting Up CritSend's SMTP relay or API

There are two ways to connect to CritSend - using our enhanced SMTP relay or using our HTTP API.
See CritSend's API doc page for API info and examples.

Our SMTP gateway is easier to use if you have an already existing system in place. Our API is better if you want a tighter integration and wish to enjoy our additional features. It offers more features but is a little bit harder to integrate with an existing system. We recommend to use our API if you build a new delivery service.

CritSend Setup Using SMTP Relay

1. Point to our relay using the following parameters:

  • Server smtp.critsend.com
  • Port 587 (Note: NOT port 25)
  • Username [your CritSend username]
  • Password [your CritSend password]
  • Encryption Plain Text
  • Amazon Web Services customers use server: aws-smtp.critsend.com

2. Begin sending messages after reading the "Message Basics & Best Practices" tutorial section

3. For Advanced Users:

SPF

If you would like to enable your own full transparency domain in your messaging, ("From: youraddress@yourdomain.com" and "signed-by: yourdomain.com") you must add SPF and DomainKeys entries to your DNS.

This can be your only SPF entry OR you can add another SPF entry and keep your existing one. If you have an SPF entry, add:
If you do not have an SPF entry and you're using BIND:
(If you are using GoogleMail your entry should look like this:)
( GoogleMail users, note that you should add both messaging-master.com and _spf.googlemail.com )

DomainKeys

You must also add this DomainKey entry to your DNS:


Messaging Basics

A few details about our enhanced SMTP server:

  • To tag your emails for in-depth link reporting for Google Analytics, add the header: X-tag. Tags must be alphanumeric only, but can contain a dash (-). For example, by adding the header X-tag: invite1 , Google Analytics will show click-thrus under the traffic source invite1. This works great for A/B testing when testing the effectiveness of different types of mail or distribution lists. If you do not add an X-tag header, your click-thrus will still be tracked, they will show up as Traffic Source: email .

SMTP Technical Details & Troubleshooting

These tips should help see you through most problems/errors:

  • Our architecture is fully redundant and distributed. If you have a 451 SMTP error or connection error in delivery, just retry the delivery.
  • We support the following SMTP authentication protocols: CRAM-MD5, Login and Plain
  • We will correctly encode the Subject automatically if it is not correctly encoded
  • Before accepting your message, we make sure that your email contains the headers From, To, Subject, and that the email is RFC compliant.
  • To activate Bcc sending capabilities, contact us.
  • If you are an advanced user and have set up your DomainKey/SenderID record in our DNS (see step #3), you can set your own domain in the Signed-by field of the Delivery Settings page. Your recipients will see your own domain in your Mail-From, Signed-by, and Sender headers if necessary, and your mail will be DomainKey/SenderID authenticated with your domains. Most powerusers only send from one domain, but if you will be sending from multiple domains, please contact us with a list of additional domains you would like us to enable.
  • If you do not wish to modify your DNS, your mails will still be DomainKey/SenderID autenticated, we will just add a Sender header with "critsend.com" in it. Your Mail-From will not be altered. For some email clients this will show up as a Mailed-by header "sent on behalf of you@yourcompany.com from critsend.com".
  • A Sender header might be added to your email if the mail you are sending contains a Mail-From with a domain unknown to us. (See above for how to enable a domain with us). For example: If in your Delivery Settings "Signed-by" field you put foo@bar.com, but you sign the Mail-From header of your email somewhere@else.com we will add a Sender header. While if you send an email from delivery@bar.com or foo@bar.com, we will not need to append a Sender header. This ensures better deliverability and authentication with major ISPs.