Is it possible to allow ICMP4 for PMTUD into a NAT'd network? As far as my understanding goes simply allowing it to the firewall doesn't do much good as the firewall isn't responsible for ...
ICMP type 3 code 3 is Destination Unreachable/Port Unreachable, it's not specific to Samba although Samba's probably popular for it because of it broadcasting to build browse lists and such.