(free) postmaster tools
Topics in this area:
enigma secure email
share your secrets via email - enigma is a secure, one-time use, passwordless link generator
inxbox temporary email
a ready-to-use mailbox that receives any message and keeps it for a limited time
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