Here's an update: HarryT was right, I re-sent the same mail via another account and within hours we got a response. Response was friendly, there was no indication whatsoever that they were aware of any prior attempts to contact them (I didn't mention in may mail that I tried to contact them before but if the had received my mails I suppose I would have been able to "read between the lines").
Info was that
"Les expéditions recommencent dès demain jeudi 24 janvier.
Les articles manquants seront expédiés en mode postal standard avant la fin
I didn't know that the delivery of missing articles was connected to the general pause in deliveries (I thought that temporary stop in deliveries was only for new orders as indicated on their page) but it seems that this bit of information was already requested by a couple of people here in the forums.
So, here's for everyone: Deliveries will start again tomorrow, 24th of January!
While I am very happy to have heard from them now, I still think that it's kind of a problem to block GMail. Not that I'm angry or something, I'm just wondering about all the people using GMail to contact some company (I suppose this problem must be the same for alle companies) and having their mails vanish into the big black void of spam protection. If these cases increase at the same rate as spam increases (and a chart of the increase of the latter would make anyone wish it was a chart of one's own shares at the stock market) that would render e-mail unusable as a means of staying in contact with one's customers. But well, that's more of a philosophical question rather than one which can really be answered at this point...
Thanks for the tip about using another account, HarryT, saved me a lot of hassle over nothing (it seems they had not forgotten anyway, just didn't tell me there was a complete stop of all deliveries in place) :-)