ok, I found that.
trick is in the NAT reflection settings.
Working config is to enable NAT reflection (either in system advanced settings, either in rule-specific settings) AND to enable tick "Automatically create outbound NAT rules…" in system advanced settings.
With this adjustment I see following packets in tcpdump (actually this is one ping packet):
13:59:07.684110 IP 192.168.0.68 > 1.1.0.1: ICMP echo request, id 512, seq 45843, length 40
13:59:07.684172 IP 192.168.0.254 > 192.168.0.10: ICMP echo request, id 29846, seq 45843, length 40
13:59:07.684299 IP 192.168.0.10 > 192.168.0.254: ICMP echo reply, id 29846, seq 45843, length 40
13:59:07.684313 IP 1.1.0.1 > 192.168.0.68: ICMP echo reply, id 512, seq 45843, length 40
and without a second tick i get:
14:00:37.735766 IP 192.168.0.68 > 1.1.0.1: ICMP echo request, id 512, seq 46099, length 40
14:00:37.735820 IP 192.168.0.68 > 192.168.0.10: ICMP echo request, id 512, seq 46099, length 40
mailserver then replies directly to my pc in local network, but it doesn't expect this echo reply…