Email testing tools

mxtoolbox.com - general testing
zerobounce.net - used to be unlocktheinbox, but has a good tool for checking your emails for compliance
checktls.com - pretty obvious
dkimvalidator.com - simple dkim, spf validator
emailsecuritygrader.com - more generalized testing of the SSL of your mail server
mailhardener.com - logging and reporting spf, fkim, dmarc, mta-sts and tls feedback loops