Very low prio, just more a feedback 
Hi, I came across very interesting interference…
I have one rule set up to automatically copy email containing in invitation to another email account and second rule to do the same in vice versa. The reason is to have invitations coming from any of those email accounts copied every time into both calendars.
The fun begun when I bought another license of eM Client and installed it to another computer and imported backup of the first instance to the new one - including these rules.
What happened is that when one instance copied invitation from one email account to another, second instance running in the same time copied the new invitation from second account back to the first one. And this loop continued until I have stopped implementing of these rules on second instance. Until than, many emails were sent from my one email account to another.
As a come out, I would like to suggest to look into this and for multi licensed users have these rules somehow synchronized and if possible to avoid such looping.
I am sure that this use case is very rare and do not expect any fixes on it any soon. I just wanted to let you know that such interference can happen and cause a quite good headache.