For some reasons the packetfilter rules for Email related ports are not defined through the package univention-mail-postfix when using Kopano.
This leaves the Submission Port 587 opened by Postfix but blocked by the packetfilter.
It might be worth to consider the following additional UCRV to be set through kopano4ucs
ucr set security/packetfilter/package/kopano4ucs/tcp/587/all/en=SUBMIT
ucr set security/packetfilter/package/kopano4ucs/tcp/587/all=ACCEPT