pfctl -s rules | grep imap (the test rule I was using), with and without disablereplyto set:
# pfctl -s rules | grep imap
pass in quick on em0 inet proto tcp from any to 192.168.56.1 port = imap flags S/SA keep state allow-opts label "USER_RULE: Special rule to test disablereplyto"
# pfctl -s rules | grep imap
pass in quick on em0 reply-to (em0 134.111.198.1) inet proto tcp from any to 192.168.56.1 port = imap flags S/SA keep state allow-opts label "USER_RULE: Special rule to test disablereplyto"