If a message has been rejected you'll see a result like that one in the screen:
By clicking on the green "eye" icon, you'll see all the details of the reject.
The most important informations are:
-Sender IP
-Sender
-Result: the reason why the message has been rejected.
Usually a reject is always a legit block.
SMTP checks are standard checks on all email communications, if a sender fails those validation most likely is a spammer.
However, if you necessarily need to handle an exception, you can move into two ways:
-Adding the "Sender IP" into Admin Area > Mail Transport > SMTP check override to completely override all SMTP checks
or
-Adding the "Sender" domain into Admin Area > Mail Transport > SPF Filtering to override only SPF check
Note: skipping those checks does not involve other security layers; Antispam, antivirus and attachments checks are still performed.