The process of testing e-mail is complicated because of the bunch of e-mail clients which you need to install on your computer. This is of course related to operating systems which you will need to install the following:
Microsoft Outlook 2007/2010/2013
Microsoft Outlook 2003/Express
Microsoft Outlook.com
iPhone Mail
Apple Mail
Gmail
Yahoo! e-mail
This bunch of e-mail clients is rather long and it is going to be problematic to test all of them. But you can use in your workflow some email testers. There is a list of online tools which you can use for example Litmus which will be described later in this chapter.