thanks for all the info, in fact I got it relatively easily to work with gmail, and meanwhile also with others, but NOT with OVH, where my exchange account is hosted... and I'd like to avoid using Gmail & Co for professional mails... so not really happy with the current setup...
I'll get my hands on a switch with a span port next week, so I'll be able to see what passes over the line (Wireshark & Co) and eventually find out why my 7830 and OVH refuse to collaborate...??? OVH support have helped as far as they could, but that doesn't go beyond setup...
I'll post my findings asap.