Like most other email providers, Port87 requires senders' servers meet minimum requirements to accept email.
You can see the results of some of these checks by clicking the "OPTIONS" button, then "Details", on a message in Port87.
Port87 uses the helo.checks plugin to make some checks against the server's HELO command.
Port87 uses the dns-list plugin to check the host's IP address against known bad sender/good sender lists.
zen.spamhaus.org (https://www.spamhaus.org/)list.dnswl.org (https://www.dnswl.org/)Port87 supports TLS and strongly encourages sending servers to use a TLS encrypted connection.
Port87 uses the headers plugin to check that the email message follows the specifications correctly before accepting the message. It uses the default configuration.
Port87 uses four methods to authenticate messages.
If a message fails both SPF and DKIM and either of the other two, it will be rejected. If a message fails both SPF and DKIM, but passes the other two, it will be accepted, but it will be marked as "spoof" and "spam".
Practically, this means that in order to send mail to Port87 users, a sending server must use either SPF or DKIM to authenticate its messages.
Port87 also separately checks for DKIM alignment. Port87 will check that the "From" address is from a domain that passed the DKIM check. It does the check in "relaxed" mode, meaning if the From address uses a subdomain of a domain that passed the DKIM check, the alignment check passes.
If the message successfully passed the DKIM check, but not the alignment check, the message will be marked as "spoof". If it also failed the SPF check, it will be marked as "spam" too.
You can choose to relax these requirements on a specific label with the "Ignore all authentication requirements for this label" option. Be aware that turning this authentication off means that anyone can spoof an email to look like it came from anyone else.
Specifically, this option ignores the MAIL FROM is Resolvable, FCrDNS, SPF, and DKIM checks. The SMTP and Message checks above are still performed.
If the message fails both SPF and DKIM, it will still be marked "spoof", but will be accepted.
A message failing DKIM alignment is almost a guarantee that the message is trying to impersonate the From address, so this setting has no affect on the alignment check.