Emails are very important for personal and business use. They check their emails on a desktop at home or work, or they check their emails on their phones or tablets while they are commuting. Therefore, testing your email in different email clients is very important.
Litmus (May 2018)
Testing the email in a email client is completely different to testing the email in a browser. Mainly Outlook software versions tend to break the email layout due to some of the CSS elements not being supportive. Campaign Monitor has a complete breakdown of the CSS support for all email clients.
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Make sure all the hyperlinks, links to the correct web pages, allowing the user to click through and increase the number of clicks in the report. If there is an image link, be very careful of how this is implemented as the user may pass by it without clicking it. Therefore, it is recommended to put text on the image to let the user know that it's a link.
Litmus is recommended using Litmus to test emails in different software including Outlook, Gmail, Yahoo Mail and etc. It also test emails in different mobile devices including Android phones and Tablets, iPads and iPhones.
Mailchimp has a preview tool that is a Litmus-powered generating screenshots of emails.
Campaigns end up in spam folders for several reasons, including the sending email address, subject line, or message content. Sendgrid explains about how to keep the email out of the spam folder.