I have several IP addresses which are persistently connecting to my mail server, but apparently not sending anything.

195.140.184.246 - duoquattuorsx.zeta.ec-cluster.com
195.140.184.245 - duoquattuorquinque.zeta.ec-cluster.com
195.140.184.248 - duoquattuorocto.zeta.ec-cluster.com

In spite of the Italian names, the IP's are in Germany.

I get very worried when strange IP's connect to my mail server for no apparent reason. I get this :-
10:28:40 415 DMN: MSG 4730 Accepted connection: [195.140.184.246] (duoquattuorsx.zeta.ec-cluster.com)
10:28:40 415 DMN: MSG 4730 SMTP session ended: [195.140.184.246] (duoquattuorsx.zeta.ec-cluster.com)

The IP's change - today I had all three 7 times in total.

Anyone have any ideas, or should I just kill them on the Firewall?

Probably some bot trying to send spam using your SMTP server. If that's true, you probably should kill 'em on firewall or configure your SMTP server for refuse unauthorized connections.