(free) postmaster tools
Topics in this area:
click.it link tracker
click.it: the link tracker with instant email feedback
email.locker
a place where email messages can be stored and retrieved when needed
enigma secure email
share your secrets via email - enigma is a secure, one-time use, passwordless link generator
inxsend fake smtp/api service
a fake smtp/api service with web gui for testing emails in applications easily
tester for spf and dkim
an online check tool to validate SPF and DKIM settings sending an email message