(free) postmaster tools
Topics in this area:
click.it generates tracked links
create tracked links with instant email feedback
enigma shares secrets securely
share secret messages, passwords, and files via email using a secure, one-time use, passwordless link
email.locker temporary archive
a place where email messages can be stored and retrieved when needed
inxsend fake smtp/api service
a fake smtp/api service with web gui for testing emails in applications easily
tester for spf and dkim validation
an online check tool to validate SPF and DKIM settings sending an email message