Mail Servers

• SendGrid
• mailx
• postfix
• sendmail
• ssmtp

• Mail Tester
• Sender ID Framework SPF Record Wizard - very nice interface to generate SPF DNS records.
• SPF Common Mistakes
• Sender Policy Framework - Project Overview
• SPF Record Syntax
• SPF Query Tool
• MX Toolbox
• Google Apps Toolbox: Check MX
• Google Apps MX record values

SPF records are DNS TXT records that verify that the server that is sending outgoing email from your domain name is authorized.

Here's a basic example where any email sent out from the domain's A record is authorized. Otherwise, do a hard fail.

Using as an example, beandog.org:

v=spf1 a -all

Authorize only servers that have MX entries in DNS (such as mail1.beandog.org, mail2.beandog.org):

v=spf1 mx -all

Find the current MX servers using dig:

dig +short mx beandog.org
0 mail.beandog.org.

Mail Tester is a great site that you can send emails to and see how it is regarded by other mail servers. It will give recommendations on how to improve SPF records, DKIM signing, SpamAssassin results, etc.

Here is an example one sent from the A2 Hosting shared account: http://www.mail-tester.com/web-yLwB8z

You can use mailx to send email directly. When using it, set the From: address properly as it would be sent:

echo testing email | mail -r [email protected] -s testing [mail-checker address]