Getting emails from the future is a classic sign of SPAM. Unfortunately, SpamAssassin only bumps the score of these emails by less than 2 points, which isn’t usually enough to get it marked as SPAM.
To fix this, you can add a rule to SpamAssassin to block emails from the future.
First, open /etc/mail/spamassassin/local.cf in a text editor, then append the following lines:
header FROM_THE_FUTURE Date =~ /20[3-9]\d-[0-1]\d-[0-3]\d [0-2]\d:[0-5]\d:[0-5]\d [A-Za-z]+/i score FROM_THE_FUTURE 20.0 describe FROM_THE_FUTURE Email from the future
Save the file, then restart SpamAssassin:
This should score emails from the future with a value of 20, automatically marking them as SPAM.